Snom D120 unreachable randomly

Snom D120 configured on TIM BUSINESS VOIP (Italian ISP), works perfectly outbound but inbound it frequently goes "unreachable", I tried on 2 different lines (same ISP though), 2 different routers but the problem persist. I bought 3 phones of this kind and ALL 3 have the same problem...

The only thing I can see in the logs that appears when an inbound call goes unreachable is: Apr 27 14:52:46.396 [NOTICE] PHN: TPL: Socket 57 idle/connect timeout

The worst thing is that it only happens sometimes... I saw that placing an outbound call fixes the problem for the next 5/10/15 minutes.

Anyone that knows how to proceed further debugging this problem?