asterisk users - Jul 2005 - No ringing using SIP or IAX phone, ringing using ZAP channel

I try to use a SIP trunk from a VOIP provider to make land to mobile 
calls. If I do these from a ZAP channel, using an analogue phone, after 
few seconds of silence (I don't like to generate fake [r]inging) I ear 
the ringing tone from the mobile operator along with any message the 
mobile operator decide to say me.  If I try to use a SIP phone (or a IAX 
phone) attached to my asterisk box, I cannot ear the ringing from the 
mobile operator nor any message the mobile operator decide to say me.

Any idea why?

Leandro