When troubleshooting issues with your phones it is crucial that a Line Test is performed to assist in identifying network issues which could affect your Quality of Service.
Using Line Test
Navigate to the Line Test Website and once there enter your email (required) you will not need to select your geo server unless you want to test multiple servers, the test will automatically select the best server based on your latency. Then press start to begin testing your network.
Understanding the Results
The speed at which you're receiving data over the internet.
The speed at which you're sending data over the internet.
The time (in ms) that it takes your network to send then receive a packet.
The difference in packet delay also described as the measurement of time difference in packet inter-arrival time.
The summary will appear at the bottom of the page after your test is complete. This will tell you your IP address, Internet Service Provider, estimated call quality, as well as let you know how many simultaneous calls your network supports based on the results.