If you phone him, he's meant to put the phone down straight away. By contacting him (and effectively asking him to break his bail conditions) you'd also risk court action against yourself, either for contempt of court or for attempting to pervert the course of justice.
A letter might be safer, since the contact would definitely only be 'one way' but, if you can find out who his solicitor is, it might still be safest to send it via the solicitor.
The people at Victim Support will probably have come across this sort of situation before, so they should be able to advise you further:
http://www.victimsupport.org.uk/
Chris