Yealink SIP-T54S Administrator's Manual page 1019

Ensure that the power is on and the network is available in the process of upgrading.
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Ensure that the web browser is not closed or refreshed when upgrading firmware via web
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user interface.
How can I verify the firmware generation and version of the phone?
Press the OK/√ key when the IP phone is idle to check the firmware version. For example:
28.81.0.20.
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Item
Firmware ID.
The firmware ID for each IP phone model is:
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28
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Firmware generation.
81 Note: The larger it is, the newer the firmware
generation is.
0 A fixed number.
Description
70: SIP-T54S/T52S
35: SIP-T48G
28: SIP-T46G
66: SIP-T48S/T46S/T42S/T41S
29: SIP-T42G
36: SIP-T41P
54: SIP-T40P
76: SIP-T40G
46: SIP-T29G
45: SIP-T27P
69: SIP-T27G
44: SIP-T23P/G
52: SIP-T21(P) E2
53: SIP-T19(P) E2
37: CP860
78: CP920
Troubleshooting
993