Aastra Telecom SIP 57I - Manual (Page 344)

Operational Features
5-134 41-001160-00, Release 2.1, Rev 04
IP Phone Administrator Guide
Operational IP Phone Features
Aastra Web UI
1 Click on Advanced Settings->Global SIP->Advanced SIP Settings.
2 Enter a value, in seconds, from 120 (2 min) to 3600 (1 hour) in the "ACD Subscription Period" field.
3 Click to save your changes.