I'm sure of two things:
You want to get this fixed the right way.
You want to get this fixed quickly, so your customer doesn't suffer.
If you want it fixed so that the provider is supplying the proper voltage and current, you can pretty much go buy a lottery ticket, and you'll have the same statistical success.
It will take lots of time to do measurements, and to arrange "vendor meets" and at the end of the day, you'll still have the problem.
To get the customer working again in a half-hour, do this:
Get a used, small, cheap, key system. Program the suspect incoming line onto a trunk port, and program a single-line station port as the input to your customer's systems. If you can get a key system that uses close to 48 volts on its SLT's all the better.
An incoming call will now get passed to the SLT port, and perhaps the customer's system will react favorably with it. Perhaps not, but it's worth a shot.
