PPM was not configured correctly in the Avaya SBCE. Personal Profile Management works through HTTPS protocol and port 443 to send the advanced CM features/capabilites to the endpoint. However, customer configured PPM in SBCE to use HTTP/port 80.
There's another cause of this problem that is the wrong certificate selected for the HTTPS, make sure that the correct certificate is used when you configured the reverse proxy, the server certificate should be the one that was issued by the root authority that you installed the root certificate in the phone.
Cause 2 - EC500 was not properly set up on the Apple and Android Equinox clients
In another instance Avaya Equinox Windows was connecting to SBC and issue was only reproducible on internal network.