Hi Gary, I believe the nat.orbtalk.co.uk:5065 is used in lieu of STUN to traverse NAT.
Since the Voxalot server itself is handling the registration with Orbtalk, you do not have to worry about NAT issues as far as connecting with Orbtalk.
As such, I do not think that the 5065 port issue indicates Orbtalk is planning to ban Voxalot on "5060" port.
See also :
Orbtalk | Support
I am using a similar service from Voiptalk, and it uses the same type of setup.
The Voiptalk website has a bit more technical information on harware setup, which kind of infers the above.