The 57i CT cordless handset must be paired to the base unit to work.  Instructions to pair the handset are detailed in the guide below.  If you need to unpair the handset, the guide will also walk you through that.

• Aastra 57i CT Pairing/Unpairing Guide (PDF)

Notes:

• A total of four handsets can be paired to a single base unit. 
• Though the base station supports up to 4 cordless handsets, only one cordless handset will work at a time.  Example, you have 2 handsets paired to the base station.  If a call is made or received through handset 1, handset 2 will not be available for use until the user with handset 1 hangs up.

Having trouble pairing or unpairing the cordless handset?

Pairing Difficulties:
If you recently unpaired the cordless handset and are attempting to pair it again (either to a new base unit or the one it was previously paired to), wait at least 24 hours before attempting to pair it again.

Unpairing Difficulties:
If you are not given the option to unpair the handset from the base unit or cordless handset, unplug the battery from handset for at least 24 hours.  Then scroll back through the phone and you should have the option to unpair.