I have successfully registered a Yealink and a Polycom phone without any issues.
I am using the provisioning module to setup the phones. I have no issues so far if I use UDP when registering the GXP2135. I can call both directions. If I try to use TLS I can call other extensions from the grandstream. It shows up in the reports as registered however when I call the grand stream extension it rings and as soon as a pick up the call, I get sent to voicemail.