"Ring...."[]
If you whack one in BS2 with your melee weapon it makes a nice ringing sound.
Its too bad they didnt have some 'use' action for these phones in both games - something like random of out-of-order recording/tone/static/busy-signal a BigDaddy or a Little Sister or a/several deranged splicer rantings on the other end, or a number of similar audio recordings.