I would be willing to bet this is a NAT issue that could be resolved with port forwarding. Dropping at a set time like that almost always is.

I think this is reinforced by what the original poster is saying in that his VOIPo configured device doesn't have the issue likely because the NAT handling is configured differently. Outside of port forwarding, you could also try enabling STUN in your device which can help with NAT issues. All of our SIP servers run STUN on them so you can use the same server you register to as the STUN server.

Different carriers that we use handle audio differently so you could have NAT related issues with calls that route through one but not another.

Another option is to contact support and have us to "proxy" your audio which means we try to force it to go through our servers as a middleman.