Aastra Telecom SISIP 57I CT - Manual (Page 308)

Advanced Operational Features
6-10 41-001160-00, Rev 01 Release 2.0
IP Phone Administrator Guide
Advanced Operational Features
Aastra Web UI
Step Action
1 Click on Advanced Settings->Global SIP->RTP Settings.
2 Enable the "Two Call Support" field by checking the check box.
(Disable this field by unchecking the box).
3 Click to save your settings.
Note: The session prompts you to restart the IP phone to apply the configuration settings.
4 Select Operation->Reset and click ..