Configures the desired line to apply the key feature. (not applicable to SIP-T19(P)
E2 and CP860 IP phones)
For line keys:
X ranges from 1 to 29 (for SIP-T48G)
X ranges from 1 to 27 (for SIP-T46G/T29G)
X ranges from 1 to 15 (for SIP-T42G/T41P)
X ranges from 1 to 21 (for SIP-T27P)
X ranges from 1 to 3 (for SIP-T40P/T23P/T23G)