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. IIRC, the maximum continuous current that any device is allowed to draw is 10 uA (?FCC Part 68?). That's federal regulations and can get you arrested and fined. . Your carrier will not allow you to attach unapproved devices. That can get your service turned off. . It can be done, but is it worth it?
In most places it's illegal to plug unauthorised devices into a phone line. The chances of them catching you are slim but.... is it worth it? What's wrong with batteries eh?
Note that Telco says NOTHING but approved equipment should be connected to the phone line. If they detect anything else on the line they're within their rights to cut off your service.
Emergency but you'll risk screwing things if you tap the phone, batteries (Lead-acid) are a bit "safer". L