Hey Donna,
In CUCM under the SX20 phone configuration check to make sure that 'Multipoint Mode' is set to 'Use Endpoint' if you want it to use the multisite feature on the SX20. If you are wanting to use a conference bridge on CUCM using an MRGL, make sure it is set to 'Use Media Resource Group List'.
If this is configured correctly it may be another setting causing the failure. We would need to see some logs to determine why.
-Chad