Yeah, sometimes it's the sim card slot, sometimes it's the telephony chips. Seems to vary by device and your luck. Either way, seems that all problems of sim-card/telephony not-working variety boil down to 'something not attached well enough to the motherboard'. Which is really annoying. But fortunately is typically fixable, as you've discovered.

Anyway, congrats on your bargain acquisition and welcome back into the fold.

I'm happy for you sir.