*****one-X Agent 2.5.8 PATCH 6 has provided this native support, part of Solution is to upgrade to latest version of software*****
The One-X Agent release 2.5.5 does not support the 96X1 in CM. It needs to be set as 96XX type set example (9630, 9640 etc)
Please Refer to the below document under the topic "IP Telephones for the Desk Phone Mode" (Page 19)
https://downloads.avaya.com/css/P8/documents/100177200
Here’s an excerpt from the document
IP Telephones for the Desk Phone mode
Avaya one-X® Agent supports the Avaya deskphone series.
The following deskphone types are available on Communication Manager and officially supported with Avaya one-X® Agent:
• Avaya 24xx series
• Avaya 46xx series
• Avaya 64xx series
• Avaya 96xx series, except 9610. The 9610 deskphone is incompatible with ACD deskphone.
Note:
Avaya 96x1 sets, aliased as 96xx sets if operating with Communication Manager before the 6.2 release.
• Clarisys i750 USB Internet Phone
Note:
You can use the 16xx series H.323 deskphones with Avaya one-X® Agent if the deskphones are provisioned as 46xx series deskphones on Communication Manager
If the set type is configured accordingly and the user type is matching with the available licenses and the problem still persists, please open a Service Request with Avaya Support for investigation.