We have three IAX phones that remotely connect to an Asterisk server. The problem is that when doing outbound calls from those phones to a cellphone (through a service provider), we do not hear any ringing tone even though the cellphone actually rings.
We also have other SIP phones that does the same but they do not encounter such problems. Furthermore, when doing outbound calls to fixed-line phones from the IAX phones, no problems with regards to ringing tones are also experienced. It is only the case of [IAX Phone -> Asterisk -> Service Provider -> Cellphone] that ringing tones can not be heard.
Any insights on this problem?