Luckily telephone lines are very current limited. You'd be hard pressed to draw more than 100mA even with a dead short at ring voltages. One zener diode might be all you need.

Also not the most unusual thing I've seen over a RJ11, which includes keyboards, RS-232, other random digital IO and full USB with data. But these are normally on items that are not phones.

Dunno about you but I don't know anyone with live RJ-11 jacks in their house anymore.

In contrast, I've been messing around with some old analog phones and VOIP stuff recently, and I've been blown away with how good the quality is over what I was expecting. The audio bandwidth is low yes (it's G.711u, so 8 KHz), but the actual quality is great, with no hyper-compression artifacts or over-applied background noise filters. And yes this is on the original 60's carbon microphones.

The regular phone calls I normally make on my cell still all sound like they are underwater for some reason, except for those rare times when the better connection is actually working.

I've seen projects that swap out the rotary phone speaker with a speaker from a pair of on-ear headphones.

The Secret Life of Machines TV show has a fun demo of how the speakers in old telephones produce sound https://youtu.be/MCgCSMq5Xpo?si=CT5aOGttBpPk9tEM&t=458

Interesting. I remember voice clarity being exceptional on old handsets and being really subpar on gsm and later. Only now we FaceTime voice and discord the quality seems to be coming back.

How can that be if the speakers in phones ubiquitously were so bad?

i wouldn't want my child making unencrypted calls :p

Mmmmmmmmmmhhh. Maybe better to damage A battery than to have a drone fall out of the sky? Also most protected cells have modest current capabilities, and might not be enough for many drone applications.

By its nature, MUAVs tend to push batteries to their limits and lifecycles tend to be limited. A battery that does mission X must have a maximum weight and a minimum capacity, and a minimum current spec. If you increase the life cycles, you decrease the maximum performance (range, payload, etc).

When you get into bigger drones, more modest restrictions on mission capability give acceptable tradeoffs, but for MUAVs short cycle life is often seen as an acceptable cost.

Frequently , after a hundred (or often much less) cycles in a MUAV a battery is retired from stressful missions due to significant drops in capability. It’s still fine for other uses, but at 90% it’s no longer sufficient for frontier MUAV applications.

The best choice to keep drones from falling out of the sky is to make sure that your auto land is set to a reasonable battery level (calibration by height if you will be at altitude) and use cell cutoffs at or below the minimum safe voltage if at all. (They are usually set to a few tenths of a volt above the minimum safe, often as much as 500mV.) The final battery voltage should be managed manually and batteries tested if over-discharged.

At least, that’s what prevents the most hardware from falling out of the sky onto people’s heads, sometimes bursting into flame on impact. Usually that is seen as the least desirable outcomes of a battery stress situation.

This project uses the same AG1171 SLIC module; it's just not enough by itself and still requires a microcontroller (or at least non trivial logic) even before you get to processing the audio streams.

There's a lot of hardware out there that does 95% of what OP's does, though you'll pay a pretty penny for it (e.g. https://www.peplink.com/products/cellular-modems-adapters/po... ) though ther's been a flood of them from noname sellers on Aliexpress and eBay under the banner of "LTE Landline Converters"

The next move is to go further back in user interfaces: To place a call, pick up the phone and dial 0 or turn the crank to ring the operator.

Have a bot do the rest, exactly like a telephone operator.

Fashion bag. Dock your cell-phone inside, bluetooth pair to the purse (w/ functional receiver + off-hook sensor) and treat that as a siri-enabled headset in a phone w/ fabric coiled cord.

"Ring ring! Hello? Hey Siri, what's the weather and call my ex so I can hang up on them!"

or in the corporate office