When testing a call to my cell, I don't hear ringing on the GXP but I see the number calling on my cell. I answer cell but no sound on either device. When I call the GXP from my cell phone, it does not ring, nor does it show up on the GXP as an incoming call. Any ideas?

Hi Jason,

Yes try setting NAT to yes on your SIP phone control whithin your PBX.

See more on this issue here: https://www.ringroost.com/how-to-set-up-and-register-a-sip-phone.php#step_4_2

That was it.