AASTRA 6700i for OpenCom 1000 User Guide EN - Manual (Page 7)

Aastra 673xi / 675xi SIP Telephones
3
Key Extensions
With an Aastra M670i or Aastra M675i key extension the Aastra 6753i, Aastra 6755i
and Aastra 6757i telephones can provide additional function keys:
Up to three key extensions can be connected to designated telephones: three key
extensions of either the M670i type or three of the M675i type. Key extensions
cannot be combined on the OpenCom 1000 communications system:
Note
The Aastra 6730i, Aastra 6731i or Aastra 6751i do not offer key extensions.
Aastra M670i: 36 function keys Aastra M675i: 20 softkeys respectively
programmable on three levels (total of
60 storage locations)
Key Extension can be connected to the telephones
(up to three) M670i Aastra 6753i
Aastra 6755i
Aastra 6757i
(up to three) M675i Aastra 6755i
Aastra 6757i