If you're comfortable designing PCBs I would definitely take that route and base it on the components I've used. Mine has a couple of irritations (not able to charge when off, voltage dropping below 5v after the battery loses a bit of power so the mic and speaker stop working, etc). Feel free to reach out if you have any questions!
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Wow, this is a project I've been kicking around for years! I have an old red rotary phone (a bit newer than yours) held aside just for it.
Definitely bookmarking your post
If you're comfortable designing PCBs I would definitely take that route and base it on the components I've used. Mine has a couple of irritations (not able to charge when off, voltage dropping below 5v after the battery loses a bit of power so the mic and speaker stop working, etc). Feel free to reach out if you have any questions!