I'm in desperate need of assistance.

My Aastra 9133i phones drop incomming calls after 4 sec of two way conversation. soft phones and a Linksys SPA-942 work just fine.

As far as I can tell the Aastras are as factory default with just the extension login credentials configured.

What have I missed?

I'm not conviced this is an Aastra problem.

The tests involved forwarding incoming calls via the Hunt Group (full of Aastras) which dropped calls vs. forwarding calls to specific extensions configured with an SPA-942 and a soft phone.

This may be a Hunt Group vs. extension problem NOT Aastra.

Anyone else having Hunt Group problems. (Firmware is 3.5.5_232201)