Yealink SIP-T4X Administration Manual page 252

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.
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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
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upgrading.
Ensure that the web browser is not closed and 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 noise, the possible reasons could be:
Users are seated too far out of recommended microphone range and sound faint,
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or are seated too close to sensitive microphones and cause echo.
Intermittent voice is mainly caused by packet loss, due to network congestion, and
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jitter, due to message recombination of transmission or receiving equipment (i.e.
timeout handling, retransmission mechanism, buffer under run).
Noisy equipment, such as a computer or a fan, may cause voice interference. Turn
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off any noisy equipment.
Line issues can also cause this problem; disconnect the old line and redial the call

to ensure another line may provide better connection.
Troubleshooting
237