Are you trying to register SX20 to WebEx or connecting SX20 to WebEx conference (WebEx is not SIP proxy server nor H323 Gatekeeper…)?
Can you describe more detail what you are looking for?
BTW, if you are looking for SX20 to register on CUCM and connecting WebEx conference as audio participant (via ISDN Gateway), it is supported but not yet as video participant.
The device is currently registered to Webex connect via sip. It's using a URI like email@example.com at a 199.x.x.x address. Can it be silumtaniously available to callmanager and webex?
SX20 support single SIP profile configuration (up to 4 proxy servers configuration).
If you are trying to register your SX20 with multiple sip server with same SIP URL (same transport, user id, password, etc.), it is possible to register SX20 in different SIP proxy servers in same time.
However you can’t register SX20 in multiple SIP servers with different SIP URL like E20 support (E20 support multiple SIP profile configuration).