VP350 Nat problem

[juanrobaina]

We have a deployment with 620 T20 behind a NAT working with a Portaone environment in the cloud, all are working fine. We have the same configuration with 20 units of VP350 (keep alive enable) and we try different timing for keep alive intervals but we are unable to receive call in those phone (we receive 480 unavailable), the only thing that resolve the issue momentarily is placing and outbound call from the phone and works for several hours, then we think the NAT is closing the port again. Please advice.