Hi
To make a Point-to-point call you can register both CTS-1300 and C-20 (needs TC5 software, currently release pending) diectly to CUCM 8.6.2 (you can natively register C-20, c-40 etc) and will be able to make calls.
or
you can register CTS-1300 to CUCM and C-20 to VCS and then can create a SIP trunk between them to make calls.
Please let us know if you need any further info.
HTH
Arun
if helpful, pls rate.