It sounds like you may be using a landline, but just in case... The last few Motorola phones I've used have an an option under security to restrict incoming calls. One of the options is "Contacts", which allows calls from anyone in the contacts list to come through, but sends all other calls straight to voicemail. I'm guessing most other phones have a similar or semi-equivalent option (on a Nokia I did it by setting a default ring of nothing, and then all contacts to the ring I wanted - it still "rang" and the caller had to wait to get to voicemail, but didn't bother me with the call). It's effective, easy to change even for temporary numbers, and in an emergency anyone can leave a voicemail (never had a telemarketer do so, only legit interactions).