a friend of mine had a similar problem ,which turned out to be down to his new digital phones ,there were to many !!!!
he had one of these sets that lets you have a phone in every room with one base unit .
i played around for a week trying to sort it for him after he had gone through all the helplines on offer without success .
then as a last resort i asked him to unplug all his phones and his bb worked .
from there it was just a matter of adding the phones back one by one ,and when we put the last phone back online the bb stopped working again .
might be worth a try !!!!