I too have been having this problem, and have had it almost since getting the phone. I have been in touch with EE in the UK to try and get it resolved, but it took a number of attempts for them to see the issue arise. Reading through a search on Google suggests that it is present in Windows Phone 8 devices and has shown up on the Nokia 920 and the HTC 8X. Having had a number of conversations with EE in the UK, it seems there is an issue with the 4G to 3G handover which is required to use the voice services. iOS 6 had a problem with it, which was fixed in the 6.1 update. Windows Phone 8 also seems to have a problem with it, but this doesn't appear to have been fixed in the Portico update.
The only fix that seems to work is to set your mobile network to use 3G as its fastest conenction, which means that you gain no benefit from the service you are paying for (in the UK at least) of 4G.
As I said, this seems to be an issue with the OS rather than the network and the fact that it is being seen globally on different handsets appears to back it up. I'd like to think that Microsoft would take note of the increasing number of complaints raised in forums, but I suspect that it won't get fixed and I'll have to continue to pay over the odds for a service I can't use.