Yealink SIP-T2 SERIES Administrator's Manual page 974

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/CP860 IP Phones
Format
952
For ext keys (only applicable to
SIP-T48G/T46G/T29G/T27P IP phones):
X ranges from 1 to 6, Y ranges from 1 to 20, 22
to 40 (Ext key 21 cannot be configured).
When assigning the following features, you do
not need to configure this parameter:
1-Conference
2-Forward
3-Transfer
4-Hold
5-DND
7-ReCall
8-SMS
9-Direct Pickup
11-DTMF
17-URL
18-Group Listening
20-Private Hold
22-XML Group
24-Multicast Paging
25-Record
27-XML Browser
34-Hot Desking
35-URL Record
38-LDAP
39-BLF List
40-Prefix
41-Zero Touch
42-ACD
45-Local Group
50-Phone Lock
56-Retrieve Park
61-Directory
66-Paging List
77-Mobile Account (only applicable to SIP
VP-T49G IP phones)
Integer