Yealink IP phone has no service, no extension number, and a fast busy tone when I try to place a call

Make sure that the phone is assigned to a user. You can do this by logging into Ooma Office Manager and following the steps under Through the Devices Menu in this knowledgebase article that discusses assigning a device to a user.

If the device is already assigned to a user, try the following troubleshooting steps:

• Restart the phone. You can do this by disconnecting the device’s power, waiting for 30 seconds, and then plugging the power back in.
• Factory reset the phone. You can do this by pressing and holding the OK button for ten seconds. Click OK when you the phone screen asks whether you wish to reset to factory setting.