Echo ( that you are hearing) is always the result of the combination of a delay (which is understandable given the conversion process in SIP), and the audio that you are sending being either too loud, or the result of bad echo suppressors at the far end. Some of your conversation comes back towards you. If there were no delay, like a landline call across town, you wouldn't notice it, in fact the sidetone in your ear is there for a reason. When you throw in a GSM network, which you have no control over, along with VoIP,then you can get some real issues.
Is the echo outbound inbound or both? If outbound only, then if there are settings, you could try to reduce the audio level you are sending to the GSM network. That way you don't overload the echo suppressors at the far end.
