Unable to select primary extension number when steps given in the link below are followed for setting up a device type in CUCM.
Cisco DevNet: SDK Web SDK - Voice and Video - CUCM Guide
Also refer screenshot given below :
Hello Omkar-
Thanks for your inquiry! In order for this to work out, check out the following things.
Please note that for this example I've used Agent001 (you can check this user out in the UCCX_EarlyRelease lab). Agent001 is an enduser, and uses a Client Services Framework Device (Jabber Voice) as his phone with a device name of Agent001. For this example, I'm going to assume that you have a device and a user already set up.
- Go to Device>Phone, and select the device in question.
- Under the owner field, set it to the user to the user you'll be setting the primary extension on.
3. Next, create, or click on the device's current extension.
4. Scroll to the bottom, and associate that extension to the user you'll be setting the Primary extension on. 
5. Go to User Management>End User and search for the user you want to update.
6. Set the controlled device to the device that has the extension you'll be setting as primary.
7. Finally - you should now see populated options for Primary and IPCC extensions.
Please note that the above information is the best practice. This is what is what will work the best. Technically speaking, I think you can just do steps 3 and 4 and you should get the options populated or Primary and IPCC extensions. For a good example, just take a look at the Agent001-Agent020 users. Please let me know if you have any additional questions!
Thanks!
Jacob
