3 to 5V? How did that work over the distances? The analog landlines in my country usually would run at 60V and then be pulled down to *I think* 14V when used. Upstream there was a resistor in series and the phone would pull a few mA when you lifted the receiver (that's how they'd register that) and you'd get a few mW out of that, which also would mean you wouldn't be able to be called if you decided to steal that electricity, as upstream would think your receiver is lifted.
Interesting thing about landlines was that (at least here) they usually were completely disconnected from the normal electricity net except upstream so even when the power in your whole neighborhood failed, the landline phones could still be operated off the power of the phone grid. (fancier touch dial phones, yes maybe even with screen and number memory and all that jazz if you were Mr. Moneybags, would lose all that function but could still be operated as phones in the most basic sense of the word, even if the dial tone would sound all weird in that scenario) Now the smartphones would still all work because they run off battery, the cell towers don't though and that'll only get worse with 5G. The old analog landlines were more failure resistant in case of catastrophe or emergency.
Anyways.. If you started draining those few mA in order to charge a battery or whatever, you wouldn't be able to be called anymore and also the phone would stop working because it needed those itself to work, if it wasn't fancy and had a power supply. then In order to be ringed that voltage also would be alternated, modulated at 25 Hz (here) (I don't know the correct english term but that literally was what made the bell ring in old phones, in newer phones that AC current would be converted to DC first and then used to feed some sound IC) so that'd also blow up your battery in a scenario where you somehow managed to shave off so little mA that both upstream didn't think you lifted the receiver and your phone still would work. I mean possible to deal with too if you're really bored but it wouldn't make much sense.
(This alternating voltage btw. was the reason the old timey phones most of you probably only saw in movies had the crank the actor would energetically crank before he started to talk, it's to generate that alternating current. No, the wheel on rotary phone had a different reason - it was really about generating the pulses for the numbers you were dialing. More "modern" phones had fancy inverters for that AC)
So, now the thought process could be, "eh, the telecom company is rich and I don't really like making phone calls anyways so I don't try to be subtle and just pull as much as I need" Yeah no that'd just get your line automatically disconnected and make some irate telecom workers show up in your neighborhood and eventually at your place. If it goes to court then the issue wouldn't be stealing electricity, but screwing with the telecom infrastructure which in most countries was and still is, like, "we're looking at prison time" illegal.
Even if you somehow managed anyways what you'd get out of the line would've paled next to one of these fancier pocket calculator solar cells or a hand crank. So yeah, it's an urban myth. Sorry.
