A way to test line levels so that they can be properly adjusted in pattons or other gateways. Some gateways have a method of doing this. It's required to have more than 1 line: The gateway (Grandstream is the only one that I know of that does this) calls line 2 from line 1 and generates a tone and then adjusts according to this. It would make more sense to have the pbx to generate the signal and make a recording of both calls and then analyze the audio and deliver the numbers to enter into the pattons or other gateways. This would eliminate calls about echo, and volume issues.