Caller ID

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Overview

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Technical Specifications

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Frequencies and Timing

• Uses Bell 202 physical layer (1200 baud)
• Special caller ID data decoder (databits_decode_callerid)
• 8 data bits per byte
• Receive-only mode (TX not supported)
• Auto-carrier detection not recommended

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Transmission Timing

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Message Format

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Data Structure

[Message Type] [Message Length] [Data...] [Checksum]

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Single Data Message Format (SDMF)

0x04 [Length] [Month] [Day] [Hour] [Minute] [Phone Number ASCII] [Checksum]

04 0E 30 35 31 38 31 30 35 35 35 31 32 33 34 56
│  │  │  │  │  │  └─ Phone: "5551234"
│  │  └─ Time: 05/18 10:55
│  └─ Length: 14 bytes
└─ Type: SDMF (0x04)

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Multiple Data Message Format (MDMF)

• 0x01 - Date/Time (MMDDHHNN)
• 0x02 - Phone Number (ASCII)
• 0x07 - Name (ASCII)
• 0x08 - Name Unavailable

80 [Length] 01 08 [MMDDHHNN] 02 0A [5551234567] 07 09 [JOHN DOE] [CS]
│           │     │           │     │            │     │
│           │     │           │     │            │     └─ Name
│           │     │           │     └─ Phone number
│           │     │           └─ Parameter: Phone
│           │     └─ Date/time data  
│           └─ Parameter: DateTime
└─ Type: MDMF (0x80)

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Parameter Types

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Special Values

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Private/Unavailable

• ‘P’ or ‘O’ - Private number (blocked)
• No number field - Unavailable

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Presentation

• Available - Number and name provided
• Private - Caller blocked ID (*67 prefix)
• Unavailable - Network cannot provide
• Out of Area - Call from outside service area

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Usage Examples

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Receiving Caller ID

# Connect phone line audio to sound card input
arecord -f S16_LE -r 8000 | minimodem --rx callerid

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Audio Interface Setup

# Many USB phone adapters work as audio devices
arecord -l  # List audio devices
arecord -D hw:1,0 -f S16_LE -r 8000 | minimodem --rx callerid

# Direct connection to phone line (with proper isolation!)
# Use a DAA (Data Access Arrangement) for safety
arecord -f S16_LE -r 8000 -c 1 | minimodem --rx callerid

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Building a Caller ID Receiver

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Hardware Requirements

1. Phone Line Interface
   • USB phone adapter, or
   • DAA (Data Access Arrangement) module, or
   • Commercial caller ID modem
   • WARNING: Direct phone line connection requires proper isolation
2. Audio Interface
   • Sound card with line input
   • 8 kHz sample rate minimum
   • Mono input sufficient
3. Computer
   • Linux with ALSA support
   • minimodem installed

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Software Setup

#!/bin/bash
# Log incoming caller ID information

LOGFILE="$HOME/callerid.log"
DEVICE="hw:1,0"  # Adjust for your audio device

echo "Caller ID Monitor Started: $(date)" >> "$LOGFILE"

while true; do
  arecord -D "$DEVICE" -f S16_LE -r 8000 -c 1 2>/dev/null | \
    minimodem --rx callerid 2>&1 | \
    while read -r line; do
      # Filter out minimodem status messages
      if [[ $line =~ ^[0-9] ]] || [[ $line =~ NAME|NUMBER ]]; then
        echo "[$(date '+%Y-%m-%d %H:%M:%S')] $line" | tee -a "$LOGFILE"
        
        # Optional: Trigger notification
        # notify-send "Incoming Call" "$line"
      fi
    done
  sleep 1
done

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Decoding Caller ID Data

def decode_callerid_sdmf(data):
    """Decode Single Data Message Format (Type 0x04)"""
    if data[0] != 0x04:
        return None
    
    length = data[1]
    month = chr(data[2]) + chr(data[3])
    day = chr(data[4]) + chr(data[5])
    hour = chr(data[6]) + chr(data[7])
    minute = chr(data[8]) + chr(data[9])
    
    # Phone number is remaining bytes minus checksum
    phone = ''.join(chr(b) for b in data[10:10+length-8])
    
    return {
        'date': f"{month}/{day}",
        'time': f"{hour}:{minute}",
        'number': phone
    }

def decode_callerid_mdmf(data):
    """Decode Multiple Data Message Format (Type 0x80)"""
    if data[0] != 0x80:
        return None
    
    result = {}
    pos = 2  # Skip type and length
    
    while pos < len(data) - 1:  # -1 for checksum
        param_type = data[pos]
        param_len = data[pos + 1]
        param_data = data[pos+2:pos+2+param_len]
        
        if param_type == 0x01:  # DateTime
            result['datetime'] = ''.join(chr(b) for b in param_data)
        elif param_type == 0x02:  # Phone Number
            result['number'] = ''.join(chr(b) for b in param_data)
        elif param_type == 0x07:  # Name
            result['name'] = ''.join(chr(b) for b in param_data)
        
        pos += 2 + param_len
    
    return result

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Technical Details

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Channel Seizure Signal

• Alerts receiver to prepare for data
• Provides bit clock synchronization
• 250ms duration at 1200 baud
• Uses Bell 202 FSK (1200/2200 Hz alternation)

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Mark Signal

• Confirms carrier presence
• Final synchronization before data
• 150ms duration
• Receiver locks onto mark frequency

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Checksum Calculation

1. Sum all bytes in message (type through last data byte)
2. Take modulo 256
3. Negate (two's complement)
4. Result is checksum byte

def calculate_checksum(data):
    """Calculate Caller ID checksum"""
    total = sum(data) % 256
    return (-total) % 256

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Bell 202 Modulation Details

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International Variations

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North America (Bellcore/GR-30-CORE)

• Between 1st and 2nd ring
• Bell 202 FSK (1200 baud)
• SDMF or MDMF format

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Europe (ETSI)

• Uses V.23 FSK (1200 baud)
• Different timing and format
• FSK Alert prior to message

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Japan

• Similar to North American
• Some format differences

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UK (British Telecom)

• V.23 FSK modulation
• Between rings or before first ring
• DTMF variant also exists

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Signal Quality Requirements

• Sample rate: 8 kHz minimum (telephone quality)
• SNR: > 15 dB for reliable decode
• Frequency response: 1000-2500 Hz
• Level: -10 to -30 dBm typical on phone line

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Troubleshooting

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No Caller ID Detected

• Is Caller ID service active on phone line?
• Audio interface properly connected?
• Correct audio device selected?
• Sample rate set to 8000 Hz or higher?

# Test audio input levels
arecord -f S16_LE -r 8000 -d 5 test.wav
aplay test.wav  # Should hear phone ring

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Garbled or Incomplete Data

• Audio level too low or too high
• Noise on phone line
• Incorrect sample rate
• Channel seizure signal missed

• Adjust audio input gain
• Use line input (not microphone)
• Ensure 8000 Hz sample rate
• Start recording before first ring

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Checksum Errors

• Indicates data corruption
• May be caused by noise
• Check audio quality and connections
• Verify phone line is working properly

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Privacy and Legal Considerations

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Advantages of Caller ID

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Identification

• Screen unwanted calls
• Identify callers before answering
• Maintain call logs automatically
• Support business call routing

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Integration

• Automated call logging
• CRM integration
• Home automation triggers
• Security monitoring

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Applications

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Home Use

• Caller identification
• Call blocking
• Call logging
• Home automation triggers

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Business Use

• Automatic call routing
• CRM integration (screen pop)
• Call center management
• Billing and accounting

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Security

• Monitor phone activity
• Detect spam/fraud calls
• Access control systems
• Alert generation

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Related Protocols

• Bell 202 - Physical layer modulation
• Bell 103 - Alternative low-speed modem

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References

• Bellcore GR-30-CORE - Caller ID Specification
• Telcordia SR-TSV-002476 - Advanced Calling Features
• ITU-T V.23 - European Caller ID modulation

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See Also

• Command Reference
• Audio Configuration
• Telephone Systems