Yealink SIP-T2 Series Administrator's Manual page 473

Ensure that the Ethernet cable is not damaged.
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Ensure that the IP address and related network parameters are set correctly.
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Ensure that your network switch or hub is operational.
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The LCD screen prompts "No Service" message when there is no available SIP account
on the IP phone.
Do one of the following:
Ensure that an account is actively registered on the IP phone at the path

Menu->Status->More->Accounts.
Ensure that the SIP account parameters have been configured correctly.
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Press the OK key when the IP phone is idle to check the basic information (e.g., IP
address, MAC address and firmware version).
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Do one of the following:
Ensure that the target firmware is not the same as the current firmware.

Ensure that the target firmware is applicable to the IP phone model.
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Ensure that the current or the target firmware is not protected.
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Ensure that the power is on and the network is available in the process of

upgrading.
Ensure that the web browser is not closed or refreshed when upgrading firmware
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via web user interface.
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Check if the IP phone is configured to obtain the time and date from the NTP server
automatically. If your phone is unable to access the NTP server, configure the time and
date manually.
If you have poor sound quality/acoustics like intermittent voice, low volume, echo or
other noises, the possible reasons could be:
Users are seated too far out of recommended microphone range and sound faint,
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Troubleshooting
461