Cisco 7940 dropping audio during calls


I have remote extensions running already on AsteriskNOW using CISCO 7940 phones and all is working fine.

Today we have taken a CISCO 7940, converted it from SCCP to SIP and run the same customised config files as my other remote extensions that work fine.

The conversion happened fine, the SIP conifg.cnf.xml file loaded fine and the phone works.

However randomly sometimes after a few seconds, sometimes after a couple of minutes, and once after 6.5 mins the audio just cuts out on the remote phone, causing the user to have to hang up and reconnect their test call to me.

It does this on any call whether rang out, or in or external.

We have checked all the settings, opened all the firewall ports and still this happens randomly.

Does anyone have any ideas what the problem could be?

Many thanks!