Yealink SIP-T46S IP Phone Displays No Service :

Symptom

Yealink SIP-T46S phone LCD display shows the message No Service. There is no dial tone, and you are unable to make or receive calls.

Yealink SIP-T46S

In cases where the phone had been properly working for a long time and is only suddenly afflicted with these error messages, power cycling your network is often able to clear it up.
Next, check if the firmware is up-to-date. Recommended Firmware is 66.85.0.5.
- Check the firmware of the phone by pressing OK.
- Another way to check the version is via the web GUI by obtaining the IP address from the phone or the Client list table from your router.
  - Log in to the phone's IP address (default user/password: admin/admin).
  - Navigate to Status > Status > Firmware Version.
If the firmware does not match the recommended you can download the firmware version here.
- Download the T46S(T48S,T42S,T41S)-66.85.0.5.rom file.
- Next, go to Settings > Upgrade > Select and Upgrade Firmware.
- Click Choose File choose the firmware file saved previously, and press Upgrade.
- The phone will reboot and update it's firmware, once done proceed to the next step.
Make sure to provision the phone to 8x8 server.
- Go to Settings > Auto Provision.
- Enter Server URL: http://sipcfg.io/cfg
- Click Autoprovision Now.
- The phone will reboot twice and once booted, will have the chance to activate.

Other helpful network troubleshooting:

Swap out the Ethernet cord connecting the phone to the network for a different one (to see if a faulty cord might be the issue).
Is the phone connected to a wall jack? If so, try connecting it directly to a networking device (router or switch).
If the phone is connected to a networking device, try plugging it into a different port on the device (to see if a faulty port is the issue).
If accessible, review the firewall/router settings and be sure to disable the following:
- SIP-ALG
- SPI
- Firewall
Try setting a static IP on the phone, which has been known to help resolve these issues.