I can not talk to the UC540, but I ran into a situation recently where the SIP gateway was sending out the private extension of the phone number instead of the full DID that was registered to the provider. The provider was then blocking call.
In our SIP debugs we saw the RDNIS information of the private extension I believe. The error code we were getting back from SP was code 404 or something along those lines.
I recommend you do some debugs and track where the calls fails, compare the SNR call versus a normal call in the debugs, and then if you still get stuck post running configs and debugs back here.
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