Thanks for your help!
I did as you suggested and tried the web call back but It simply timed out with the "We're sorry. We are unable to complete your calls." message
I can't understand why my Voxalot registered Incoming numbers with Sipgate and Coms.Com are working fine with my device, but the unregistered call to my voxalot account dosn't ring.
It has worked in the recent past, but it seems to have simply stopped working.
I have recently changed the server I use for my device to the US server 64.34.173.199. and of course Voxalot has upgraded, but I can't think of anything else that may have changed.