How Do Spam Callers Spoof Phone Numbers Step by Step?

How Do Spam Callers Spoof Phone Numbers Step by Step?

Published: June 19, 2026 | Category: Fraud Awareness & Telecommunications | Level: Technical Beginner

Executive Summary

Caller ID spoofing is the practice of deliberately falsifying the information transmitted to your phone's caller ID display to disguise the caller's true identity. Understanding how this technology works is crucial for recognizing and defending against vishing (voice phishing) attacks, telemarketing fraud, and social engineering attempts.

Key Insight: Spoofing is not inherently illegal—legitimate businesses use it for privacy (doctors calling patients from personal phones) and call centers displaying main numbers. However, criminals exploit this capability to impersonate banks, government agencies, and trusted organizations.

Part 1: Understanding the Technology

What is Caller ID?

Traditional Phone System:Caller → Local Exchange → Long Distance → Recipient's Exchange → Phone Display

↓

[ANI/Caller ID Data]

ANI (Automatic Number Identification): The actual billing number

Caller ID: What displays on recipient's phone

The Vulnerability: These two numbers can be different, and the Caller ID field is not authenticated in traditional phone networks.

The Shift to VoIP

The proliferation of Voice over Internet Protocol (VoIP) has made spoofing exponentially easier:

Part 2: Step-by-Step Spoofing Process

Method 1: VoIP Service Spoofing (Most Common)

Step 1: Obtain VoIP Account

Requirements:- Sign up with VoIP provider (Twilio, Vonage, RingCentral)

- Some require minimal verification

- Others allow anonymous sign-ups with crypto

- Cost: $10-50/month for basic service

Legitimate Providers:

- Twilio

- Vonage Business

- RingCentral

- 8x8

- Nextiva

Illicit Providers:

- Sold on dark web forums

- No verification required

- Pay with cryptocurrency

- Designed specifically for spoofing

Step 2: Configure SIP Trunk

python

# Example SIP Configuration (Educational Only)# Session Initiation Protocol settingsSIP Trunk Configuration:- Server: sip.provider.com- Username: [Account ID]- Password: [Authentication Token]- Outbound Caller ID: [SPOOFED NUMBER HERE]- Display Name: [FAKE NAME]Technical Details:Protocol: SIP (Session Initiation Protocol)Transport: UDP/TCP/TLSPort: 5060 (standard), 5061 (TLS)Codec: G.711, G.729

Step 3: Set Custom Caller ID

Provider Dashboard Settings:

Caller ID Settings:

├─ Outbound Caller ID: +1-555-0199

├─ Display Name: "IRS Tax Department"

├─ Validated: [False - many providers don't verify]

└─ Emergency Caller ID: [Real number for 911]

Advanced Settings:

├─ CNAM (Caller Name): "Bank of America"

├─ SIP Header Manipulation: Enabled

└─ Custom P-Asserted-Identity: [Spoofed Number]

Step 4: Place the Call

Call Flow:

1. Spoofer initiates call via VoIP softphone

↓

2. Call routed through VoIP provider's servers

↓

3. Provider inserts custom Caller ID into SIP headers

↓

4. Call enters PSTN (Public Switched Telephone Network)

↓

5. Recipient's carrier receives call with spoofed ID

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6. Phone displays fake number to victim

Technical Breakdown:

sip

# Actual SIP INVITE message (simplified)

INVITE sip:[email protected] SIP/2.0

Via: SIP/2.0/UDP spoofers.ip:5060

From: "IRS Tax Department" <sip:[email protected]>

To: <sip:[email protected]>

Call-ID: spoofed-call-12345

CSeq: 1 INVITE

Contact: <sip:[email protected]>

P-Asserted-Identity: <sip:[email protected]>

Privacy: none

Content-Type: application/sdp

# The "From" field contains the spoofed number

# P-Asserted-Identity is trusted by carriers but can be manipulated

Method 2: Spoofing Apps (Easiest)

Step 1: Download Spoofing Application

Popular Apps (Educational Awareness):- SpoofCard

- SpoofTel

- Bluff My Call

- Caller ID Faker

- Fake Caller

Availability:

- iOS App Store (some restricted)

- Google Play Store (many removed)

- Direct APK downloads

- Web-based services

Step 2: Account Setup

Registration Process:1. Download app or visit website

2. Create account (email/phone)

3. Purchase credits ($5-50)

4. Some accept cryptocurrency

5. No identity verification required

Step 3: Configure Spoof

App Interface:

Caller ID Settings:

├─ Enter number to call: [Victim]

├─ Display number as: [Spoofed Number]

├─ Voice changer: [Optional - disguise voice]

├─ Record call: [Optional]

└─ Background sounds: [Police station, office, etc.]

One-Click Spoof:

[Call Now Button]

Step 4: Execute Call

Behind the Scenes:

1. App connects to VoIP gateway

2. Sends spoofed Caller ID in SIP headers

3. Routes through multiple carriers

4. Displays fake number to recipient

5. Records call if enabled

Method 3: PBX/System Manipulation (Advanced)

Step 1: Access Corporate PBX

Attack Vectors:- Compromised credentials

- Exploiting vulnerable PBX systems

- Insider access

- Social engineering IT staff

Target Systems:

- Avaya PBX

- Cisco Unified Communications

- Asterisk (open source)

- 3CX Phone System

- Mitel

Step 2: Modify Outbound Rules

asterisk

; Asterisk PBX Configuration Example; Educational - Understanding vulnerabilities

[outbound-spoof]

exten => _X.,1,NoOp(Spoofed outbound call)

exten => _X.,n,Set(CALLERID(num)=15550199)

exten => _X.,n,Set(CALLERID(name)=IRS)

exten => _X.,n,Dial(SIP/trunk/${EXTEN})

exten => _X.,n,Hangup()

; This Asterisk dial plan sets custom Caller ID

; before routing through SIP trunk

Step 3: Route Calls

Call Routing:

Compromised PBX → SIP Trunk → PSTN → Victim

↓

Custom Caller ID injected at PBX level

Appears to come from legitimate business

Part 3: Technical Deep Dive

How SIP Headers Enable Spoofing

SIP Message Structure:

┌─────────────────────────────────────┐│ SIP INVITE Message │

├─────────────────────────────────────┤

│ Via: Routing information │

│ From: [SPOOFED NUMBER HERE] │

│ To: Destination number │

│ Call-ID: Unique identifier │

│ Contact: Callback address │

│ P-Asserted-Identity: [Trusted ID] │

│ Privacy: [Caller preferences] │

└─────────────────────────────────────┘

Key Headers Exploited:

The Trust Problem

Traditional Telephone Network:┌──────────┐ ┌──────────┐ ┌──────────┐

│ Caller │───→│ AT&T │───→│ Verizon │───→ Victim

│ (Real) │ │ (Trusts) │ │ (Trusts) │

└──────────┘ └──────────┘ └──────────┘

VoIP/Internet Age:

┌──────────┐ ┌──────────┐ ┌──────────┐ ┌──────────┐

│ Spoofer │───→│ VoIP │───→│ PSTN │───→│ Victim │

│ (Fake) │ │ (No │ │ (Trusts │ │ (Sees │

│ │ │ Verify) │ │ VoIP) │ │ Fake) │

└──────────┘ └──────────┘ └──────────┘ └──────────┘

Problem: No end-to-end authentication

Part 4: Identifying Spoofed Calls

Technical Indicators

Red Flags Checklist

□ Caller claims to be government agency (IRS, SSA, FBI)□ Demands immediate payment via gift cards/wire transfer

□ Threatens arrest or legal action

□ Requests sensitive information (SSN, passwords)

□ Caller ID shows local number but wrong time zone

□ Robocall with urgent message

□ Caller refuses to provide callback number

□ Number returns "not in service" when called back

Part 5: Prevention and Defense

For Individuals

Immediate Actions:

1. Don't Trust Caller ID - Treat all unsolicited calls as suspicious

- Verify independently through official channels

2. Hang Up and Call Back

- Use official website phone number

- Not callback number provided by caller

3. Enable Call Filtering

- iOS: Settings > Phone > Silence Unknown Callers

- Android: Phone app > Settings > Caller ID & Spam

4. Use Third-Party Apps

- Hiya

- Truecaller

- Nomorobo

- RoboKiller

For Organizations

Technical Defenses:

STIR/SHAKEN Explained:

STIR (Secure Telephone Identity Revisited)SHAKEN (Signature-based Handling of Asserted Information)

How It Works:

1. Originating carrier authenticates caller

2. Digital signature added to SIP headers

3. Intermediate carriers verify signature

4. Receiving carrier validates and displays status

Result:

✓ "Verified" - Legitimate caller

⚠ "Partial" - Some authentication

✗ "No Verification" - Potential spoof

Part 6: Legal and Regulatory Framework

Laws Against Spoofing

Enforcement Challenges

Problem 1: International Calls- Spoofer in India → Victim in USA

- Jurisdiction issues

- Limited cooperation

Problem 2: VoIP Anonymity

- Easy to hide identity

- Prepaid services

- Cryptocurrency payments

Problem 3: Legitimate Uses

- Doctors calling patients

- Call centers displaying main numbers

- Privacy protection

- Hard to distinguish from fraud

Part 7: Case Studies

Case Study 1: The "IRS" Scam (Ongoing)

Attack: Mass vishing campaign spoofing IRS phone numbers
Method: VoIP spoofing + robocalls + live agents
Loss: $65 million annually (estimated)
Target: Taxpayers during filing season

Technical Details:

• Spoofed 202-555-0199 (Washington DC area code)
• Displayed "IRS" as caller name
• Threatened immediate arrest
• Demanded gift card payments

Case Study 2: Bank Fraud (2024)

Attack: Spoofed major bank customer service numbers
Method: Compromised PBX + social engineering
Loss: $12 million
Target: High-net-worth individuals

Technical Details:

• Hacked small business PBX
• Used legitimate bank numbers
• Transferred funds after "verification"
• Calls appeared completely legitimate

Case Study 3: Healthcare Scam (2023)

Attack: Spoofed hospital and insurance numbers
Method: VoIP service + purchased patient lists
Loss: $8 million + compromised PHI
Target: Medicare/Medicaid patients

Technical Details:

• Spoofed local hospital numbers
• Offered "free" medical equipment
• Collected insurance information
• Billed for services never rendered

Part 8: The Future of Call Authentication

STIR/SHAKEN Implementation

Timeline:2019: FCC mandates implementation

2021: Major carriers deploy

2024: Full intercarrier implementation

2026: International expansion

Current Status:

- 80% of US calls have authentication

- International remains challenge

- Small carriers lagging

- Spoofing still possible but harder

Emerging Technologies

Quick Reference: Spoofing Detection Guide

Questions to Ask

When you receive a suspicious call:

1. "Can I call you back at your official number?"

- Scammer: Will refuse or give different number

- Legitimate: Will agree and provide verified number

2. "What is your employee ID and extension?"

- Scammer: Will make up information

- Legitimate: Will provide verifiable details

3. "Can you send this request in writing?"

- Scammer: Will pressure for immediate action

- Legitimate: Will comply with written follow-up

4. "What is the last four digits of my account?"

- Scammer: Won't have it or will guess

- Legitimate: Will verify with partial information

Verification Steps

Step 1: Hang up immediatelyStep 2: Find official number (website, statement)

Step 3: Call official number directly

Step 4: Verify the request

Step 5: Report if fraudulent

Conclusion

The Bottom Line

Caller ID spoofing is:

• Technically simple with modern VoIP
• Widely available through apps and services
• Difficult to prevent completely
• Used by both legitimate businesses and criminals

Your Best Defense:

• Never trust caller ID alone
• Always verify independently
• Hang up on suspicious calls
• Report spoofing attempts

Remember

If it sounds too good to be true → It isIf they're pressuring you to act now → Hang up

If they demand unusual payment methods → Scam

If you didn't initiate the contact → Verify

Tags: #CallerIDSpoofing #Vishing #PhoneScams #FraudAwareness #STIRSHAKEN #Telecommunications #Security

Stay informed. When in doubt, hang up and verify through official channels.