Phone Number Validation Best Practices

Validation Levels

Phone validation operates at multiple levels, from simple format checks to carrier-level verification:

1. Format Validation

Check that the number follows correct formatting:

• Length — US numbers are 10 digits (+ country code)
• Country code — +1 for US/Canada
• Area code — Valid NPA (area code)
• Invalid prefixes — No 555-01xx (reserved for fiction)

# Format validation example
import re

def is_valid_format(phone):
    # Remove non-digits
    digits = re.sub(r'\D', '', phone)

    # Check length (10 or 11 with country code)
    if len(digits) == 11 and digits[0] == '1':
        digits = digits[1:]
    if len(digits) != 10:
        return False

    # Check for invalid area codes
    area_code = digits[:3]
    if area_code.startswith('0') or area_code.startswith('1'):
        return False

    return True

2. Validity Checking

Verify the number exists in carrier databases:

• LRN lookup — Confirms number is assigned to a carrier
• Active status — Number is not disconnected
• Line type — Wireless, landline, or VoIP

3. Reachability Verification

Confirm the number can receive communications:

• HLR lookup — Check if SIM is active (wireless)
• Line status — In service vs. disconnected
• Roaming status — Currently reachable

When to Validate

At Data Entry

Validate numbers as users enter them:

• Form submission — Real-time format check
• Registration — Verify before sending OTP
• CRM import — Batch validate new contacts

Before Outreach

Re-validate before campaigns:

• SMS campaigns — Remove landlines and disconnected numbers
• Voice campaigns — Verify numbers are still active
• Lead follow-up — Check before sales calls

Periodic Refresh

Numbers change over time:

• Disconnections — ~15% of numbers churn annually
• Porting — Carriers change
• Reassignment — Numbers get recycled

Why Line Type Matters

Different line types have different capabilities:

Implementation Example

# Complete validation workflow
def validate_phone(phone):
    # Step 1: Format validation (free, local)
    if not is_valid_format(phone):
        return {"valid": False, "error": "Invalid format"}

    # Step 2: Carrier/validity lookup (API call)
    result = veriroute_lookup(phone, lrn=True)

    if not result.get('lrn'):
        return {"valid": False, "error": "Number not found"}

    # Step 3: Check line type for SMS capability
    line_type = result['lrn'].get('line_type')
    sms_capable = line_type in ['wireless', 'voip']

    return {
        "valid": True,
        "carrier": result['lrn']['carrier'],
        "line_type": line_type,
        "sms_capable": sms_capable,
        "ported": result['lrn'].get('ported', False)
    }

Best Practices

1. Validate early — Catch bad numbers at entry
2. Check line type — Filter landlines from SMS lists
3. Use standardized format — Store in E.164 (+15551234567)
4. Re-validate periodically — Numbers change over time
5. Batch validate imports — Clean lists before adding to CRM
6. Handle edge cases — Toll-free, short codes, international