Some needed details in your end:
1. What is the firmware version of your SX20?
2. What is the model and also firmware version of the far end endpoint used?
3. Are you doing point to point or multipoint call?
4. What type of call protocol used? H323 or SIP? or direct IP dialing?
5. Is there a call control in used? (VCS/CUCM)?
6. Is there a MCU involved in the call?
7. Is there a firewall in between your endpoint and the far end?
regards,
Acevirgil