Almost sounds like your router's NAT is closing up on your connection. Have you tried putting a STUN server setting in your adapter? I don't have your adapter model. But most, if not all, have the ability to set a STUN server. Also, some VOIP adapters have a keep-alive interval setting that can be used to send out a periodic blank UDP packet to the SIP server to keep the "hole" in the NAT open.
The Grandstream HT-286 devices that VOIPo will be using have the ability to set either one (STUN or keep-alive interval). I found that with my router, the keep-alive interval setting is what works...as the STUN setting on the HT-286 causes a 2-3 second delay hearing the other party when you answer (I do not have the same STUN problem with the HT-502, so it must be firmware related).
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