If you're using LLV Validation on your phone number field, the following list of rejection codes could be useful in determining what is wrong or how it was accepted:
110: Not Accepting Incoming Calls
120: Phone Switched Off
150: Assumed Mobile
20: Number Dialled has not been recognised
200: Call Barred
21: Live
24: Number Dialled has not been recognised
25: Live
28: Number unobtainable Tone or message
29: Live
300: Dead Number
310: Allocated to non-BT provider, likely dead
32: Network Congestion tone/message
33: Live, Call rejected
34: Ambiguous result - manual dial test required
35: Changed Number announcement message
38: Number Not available message
40: Invalid telephone number dialled
400: Bad Syntax
42: Probably dead
44: Long/short busy tone
46: Temporarily Out of Service message
47: Live
50: Long/Short busy Tone
500: Number Changed Message
52: Ambiguous result - manual dial test required
53: Ambiguous result - manual dial test required
54: Badly formatted number
55: Live
57: Live
58: Ambiguous result - manual dial test required
59: Live
600: Retry
61: Live
610: Number Unobtainable
62: Long/Short Busy Tone
620: Number Out Of Order
63: Ambiguous result - manual dial test required
630: Mobile Phone Temporarily Unavailable
640: Network Congestion
65: Live
650: Temporary Equipment Failure
660: Mobile Phone Temporarily Unavailable
67: Live
69: Live
700: Unobtainable
71: Live
73: Live
75: Live
API Problem
Client Rejected