I use VVX1500 to register to Avaya Session manager, it fail. Avaya response "401 unauthorized", and "400 Bad Authorization or Proxy-Authorization header "
Any interoperability testing would have been done by Avaya to ensure our Phones are compatible with their SIP PABX.
I must admit I have not personally seen a VVX1500 running such an old software.
Upgrading this VVX1500 would be your first task but there are certain caveats.
The Feature Descriptions & Technical Notifications for SoundPoint IP, SoundStation IP, Polycom VVX 1500, SSDuo Phones and SL8400 Handsets page => here <= contains the Technical Bulletin 53522 Upgrading the Polycom® VVX™ 1500 Phone to SIP 3.2.2 => here <=
Once you upgraded to this Software I suggest you upgrade to UC Software 5.1.2
You can actually purchase a H.323 license as this VVX most likely is not a VVX1500D which has a Dual Stack SIP / H.323 license already present.
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