Yealink SIP-T54S Administrator's Manual page 1041

of line key (line key is not applicable to SIP-T19(P) E2/CP860/CP920 IP phones), programable
key and ext key (ext key is only applicable to SIP-
T54S/T52S/T48G/T48S/T46G/T46S/T29G/T27P/T27G IP phones).
The following table lists the number of DSS keys you can configure for each phone model:
Phone Model
SIP-T54S
SIP-T52S
SIP-T48G/S
SIP-T46G/S
SIP-T42G/S
SIP-T41P/S
SIP-T40P/G
SIP-T29G
SIP-T27P/G
SIP-T23P/G
SIP-T21(P) E2
SIP-T19(P) E2
CP860
CP920
The programable key takes effect only if the IP phone is idle.
Note
The ext key takes effect only if the expansion module is connected to the IP phone.
Line Key Programable Key
27 13
21 11
29 13
27 13
15 11
15 11
3 11
27 14
21 14
3 11
2 11
/ 11
/ 8
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Appendix
Ext Key
60
60
40
40
/
/
/
39
39
/
/
/
/
/
1015