If you’re using sip to connect to callwithus then you need to make sure that you’re using the generic address sip.callwithus.com as your proxy/registrar.
From May 1st the other, more specific addresses, that used to work will stop accepting registrations (west.callwithus.com, east.callwithus.com, uk.callwithus.com)
See here for further details – http://www.callwithus.com/configuration