[Asterisk-Users] No ringing

[Ted Morgan]

We are having problems when people call us the caller hears no rings on the
phone.  We hear the phones ring in the office, but the caller thinks they have
a dead call and just keeps calling back.  It doesn't happen all the time but
is consistent from the same numbers.  If I call from one service provider
(verizon), I hear the ring, but if I call from another (Cingular) it doesn't.  

Any ideas what could be going on?

Thanks