baxtards..
I don't suppose you could use an answering machine, with a blank outgoing message, and tell your mother to leave the answerphone to handle the call when an unlisted number appears.
If it's someone that needs to get through to your mother, she'll hear the voicemail (if you can get an answerphone with a speaker) and she can determine whether to ignore the call or pick up the handset and take the call.
This way you can gather evidence, for a nuisance complaint, when the phone pest does call and you can use it as grounds for a nuisance complaint to the police.

** [edit] as minimalist was saying.. I probably replied to the wrong part of the thread **
(sarcasmo needs more sleep, Wed 9 Sep 2009, 16:33, Reply)