Yealink SIP-T54S Administrator's Manual page 730

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
The digit 35 stands for the key type URL Record.
For line keys:
X ranges from 1 to 29 (for SIP-T48G/S)
X ranges from 1 to 27 (for SIP-T54S/T46G/T46S/T29G)
X ranges from 1 to 15 (for SIP-T42G/T42S/T41P/T41S)
X ranges from 1 to 21 (for SIP-T52S/T27P/T27G)
X ranges from 1 to 3 (for SIP-T40P/T40G/T23P/T23G)
X ranges from 1 to 2 (for SIP-T21(P) E2)
For ext keys:
For SIP-T54S/T52S:
X ranges from 1 to 3, Y ranges from 1 to 60.
For SIP-T48G/T48S/T46G/T46S:
X ranges from 1 to 6, Y ranges from 1 to 40.
For SIP-T29G/T27P/T27G:
X ranges from 1 to 6, Y ranges from 1 to 20, 22 to 40 (Ext key 21 cannot be configured).
Example:
linekey.2.type = 35
Default:
For line keys:
For SIP-T48G/S IP phones:
The default value of the line key 1-16 is 15, and the default value of the line key 17-29 is 0.
For SIP-T54S/T46G/T46S/T29G IP phones:
The default value of the line key 1-16 is 15, and the default value of the line key 17-27 is 0.
For SIP-T52S IP phones:
The default value of the line key 1-12 is 15, and the default value of the line key 13-21 is 0.
For SIP-T42G/S IP phones:
The default value of the line key 1-12 is 15, and the default value of the line key 13-15 is 0.
For SIP-T41P/S IP phones:
The default value of the line key 1-6 is 15, and the default value of the line key 7-15 is 0.
For SIP-T27P/G IP phones:
The default value of the line key 1-6 is 15, and the default value of the line key 7-21 is 0.
For SIP-T40P/T40G/T23P/T23G/T21(P) E2 IP phones:
The default value is 15.
For ext keys:
For SIP-T54S/T52S IP phones:
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Parameters Permitted Values Default