Yes, SX20 can be registered with callmanager ( version 8.6 or higher and might require a device pack in some cases ).
You can check the admin guides for SX20 ( CE as well as TC based )
Yes, video call will be possible between these two endpoints whether they are both registered to same cucm or if the SX0 is standalone. It is however better to register SX20 to cucm unless you have a compelling reason to use it as standalone.
Couple of posts that you can refer
For IP dialing from cucm you can refer the following
- Do rate helpful posts -
if SX20 is working standalone that means it can only receive IP calls. so the remote device(8845) must be able to dial IP Address.
you would require EXPC to get this job done. integrate CUCM with EXPC. and then you will be able to call SX20 via its IP Address.
on EXPC. you can get this done in 2 ways.
1. by setting unknown ip addresses to "DIRECT"
2. by defining a local subzone and add the IP address or subnet which SX20 belongs to.
and creating a Search Rule of " any IP address " and pointing it to local zone.
in your case 1st method would b easier if you have no previous info about EXPC.
please rate and mark answered if helpful.