You need to invent a dial-tone detector, similar in scope and application to the stutter dial tone detectors that alert the user that a voice mail message is waiting.
But there are bigger issues than just detecting line current or voltage, neither of which guarantees that there is a live POTS line. The alarm companies with whom I deal have needed to be told about "left in" dial tone, which does almost nothing, and "left in" battery, which does even less.
Some LEC's, upon terminating service (generally for a seasonal subscriber) will leave an active dial tone at the premises. This dial tone identifies properly using ANA, but can only call toll-free numbers and 9-1-1. The reason is so that when the seasonal subscriber returns in the spring to reestablish service, they can just pick up the phone and dial the toll-free number for the business office.
Savvy alarm companies use a toll-free number for their panels to call the central station, so alarm protection is not compromised during the off-season, and the subscriber need not continue to pay for a line with full service.
So, my advice is to understand all the possibilities before even trying to narrow down a range of amperage that will be useful.
