The phone may not be getting a network connection and needs to be rebooted to reset the connection OR the phone may need to be replaced.
First, try switching out with another phone to see it can get a dial tone. If another phone does get a dial tone then your phone may need to be replaced.
Second, if switching out the phone still does not provide a dial tone then try restoring the connection:
1. Unplug/replug the ethernet cable from the back of your phone (pictured below) which is easier to reach.
2. If that does not resolve the issue unplug/replug the ethernet cable from the wall port.
10/100 SW port hole is for the Phone Connection.
If your phone issue is not resolved with these suggestions
please submit a service request.