Steve Garcia in BYTE (1983) had a DIY acoustically coupled modem ($20 in parts )

I was (in the UK) sorely tempted to change a couple of components and get our electronics shop to build a batch for a project we where doing to avoid the huge GPO tax £300 for answer and £650 for answer origionate.

All Illegal of course - though that didn't stop us hacking the modems we did buy to have variable gain.

For information we brought 6 modems (more than my salary at the time)