Yealink SIP-T54S Administrator's Manual page 66

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
To connect the expansion PSTN box:
For CP860：
Insert the USB plug on the expansion PSTN box into the USB port on the phone.
1.
A PSTN box:
Two PSTN boxes:
For CP920：
Insert the USB plug on the expansion PSTN box into the USB port on the phone.
1.
A PSTN box:
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