I would like to spam trap calls from telemarketers, etc. I can actually do this in many cases because a lot of telemarketers use easily identifiable numbers. However they do also sometimes withhold their numbers. I cannot routinely block withheld numbers as I also receive a large number of calls from genuine uk businesses who very unhelpfully also choose to withhold caller id. I recently read that when a caller withholds caller id this does not in itself prevent id information being sent, it just sets a flag saying the entity providing the DID (Direct Inward Dialing) service should strip the information before sending it on to the customer receiving the call. It also appears that some DID providers for SIP lines do not do this. I'm with Sipgate which does appear to strip the information. Does anyone know of alternative DID providers who do not strip the information.