Why do some men make obscene phone calls?

I assume you  mean men that do this on a fairly regular basis.  If a guy does this for kicks one time, that is usually immature horse play. However to do this frequently then you are talking about behavior that is a problem (at least for others).  Men who do this have a compulsion (its sort of like drug abuse). The very habit becomes repetitive and addicting.  Also addicting in the sexual gratification.  Men who are compulsive achieve stimulation from hearing the woman's voice or by touching themselves.  This level is often accompanied by masturbation.  Secondly addicting is the ability to engage an unsuspecting female into a sort of fantasy sex type situation.  Fantasy can also play a part.  The act of violating someone's privacy and their very participation in this makes it stimulating.  In our field, we call this sort of a detached way of finding sexual gratification.  It is more accepted than rape or other things, but the excitement is in involving another person into a sexual act who is not a willing participant.