I had this exact same issue with an SX80 and it turned out to be an issue with the server certificate on the EXP-E. For example, if the MRA/Service Domain for the endpoint is "abc.com" then the EXP-E server needs to have "collab-edge.abc.com" as a SAN on the certificate.
I generated a new CSR, adding "collab-edge.abc.com" as a SAN, got the certificate from the CA, uploaded it to the EXP-E, rebooted, and verified the SX80 now registers successfully via Expressway/MRA.
Configuring Cloud Connected PSTN (CCP) – Easy as 1-2-3!
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