Aastra Telecom 6757I CT - Manual (Page 160)

150 Model 6757i CT IP Phone User Guide
Line Keys and Softkeys
Expansion Module Keys
5. Select from “Key 1” through “Key 36” (M670i)
or
Select from “Key 1” through “Key 60” (M675i)
6. In the "Type" field, select Empty.
7. Click to save your changes.