Yes, sidetone (hearing your own voice through the handset) makes all the difference! I'm not exactly sure why, but I find it so much easier to talk on the phone when there's a sidetone. In general, for me, talking on the phone is like talking into the void, and sidetone at least gives me the assurance that, yes, it's still working. Like the little inset video feed of yourself when you're on a Skype call.

Yeah, exactly. Plus, I've found I'm much louder when there's no sidetone, which is very tiring.

Nice work! Maybe I missed this, but have you considered simulating a dial tone?

I haven't, I think it would be a fair bit of work, given that the modem is what outputs the sound.

I haven't worked with the Arduino. Would it be difficult to incorporate an SPDT relay to switch the sound source between it and the modem?

Hmm, probably not, actually, that's a good idea. It was one of my first projects, so I didn't really know what I was doing, which limited how much I could do.

It's a great project! Thanks for sharing it.

If the relay works you could also simulate the pulse dialing signals with clicking audio on the dial return.