I have a UC520 with version uc500-advipservicesk9-mz.124-11.XW6
The system has a sip service provided by my ISP and this service works a treat for both Incoming calls and outgoing calls.
I can receive a call from the SIP service and then transfer the call externally manually no problem.
The problem is with call forward all to an external number. I receive a recorded message from my ISP stating "your call cannot be completed at this time. please try again later"
I've contacted the ISP and they aren't seeing any traffic hitting their SIP server which to them means that an invalid number of digits are being sent.
I've checked debugs and can see no major differences between a direct call out and a call forward call out.
Voice part of the config is attached.