Polycom extension can make calls but not receive

I’m having an issue getting Polycom handsets to receive calls from my FreePBX setup. I’m also using X-Lite (a softphone) which is not experiencing this issue.

I have set up FreePBX for a small office phone system. I’m using a Digum TDM800P. I have configured 6 incoming/outgoing trunk lines. I have set up 2 extensions using the FreePBX UI. For the most part the basic setup seems to work as expected.
I have a an X-Lite SIP softphone that I use on my PC which I am able to connect to FreePBX server and register as an extension (e.g. 203). From X-Lite I’m able to make calls to outside numbers, use voice mail and conference features and receive calls from the outside and from other extensions(i.e. the phone rings)

I have set up a Polycom SP500 phone using Polycom’s SIP 2.0.1 and BootROM 3.2.2. It is booting to a TFTP server and being configured via configuration based on the Polycom sample files. The phone registers with FreePBX as an extension (e.g. 204). I am able to use it to call outside lines, use voice mail and conference features. I’m also able to dial other extensions. I CANNOT however receive any calls from other extensions or from outside lines. Basically, when I call this extension it goes right to voicemail as if the extension is not registered.

At this point, I am stumped. The X-Lite extension works perfectly fine however, the Polycom ones do not. My suspicion is that I the Polycom phones are missing some configuration option required to work with FreePBX or are miss configured. Has anyone here experienced a similar issue and if so how would you recommend going about solving it.