I did a temp change and put one client via voip.co.uk. And can produce similar call drops, around one in every 40ish. Clearly this starts to indicate the problem might not be Gradwell related but due to the increased number of drops and no change in the config for the Gradwell trunks something doesn't add up. It is also my understanding that voip.co.uk and Gradwell both use Gamma Telecom for termination.

I am aware Gradwell had some issues and had what i was told T1 line cards replaced a little while ago.

When calls drop the Asterisk PBX reports 'maximum retires exceeded to critical request'. Sounds like some packets are not received / replied to.

Although the NAT and port forwarding looks correct and has worked in the past I am trying removing NAT from the config and putting the Asterisk PBX on a pubilc IP.

Will let you know how I get on.