SIP Registration timeouts

Hello Community, I hope someone has an idea of what is causing this issue.

I suspect it is some sort of nat timeout issue, but can't find any documentation to tell me how to check what the timeout currently is, and if it can be changed, or if you can enable a keep-alive etc.

Problem is, sip handsets on the lan, are going through the cyberoam (10.6.6) to the sip server on the wan. 

Handsets randomly de-register and go "idle".

Provider says this is due to the handset losing the connection with the sip server.

If we replace the cyberoam with a basic modem/router - this issue vanishes.  Handsets stay permanently registered.

It's really frustrating.  Anyone seen this before, or can advise on a solution?

Thanks in advance.