Weird, phone-based coincidences incoming...
Found a voicemail notification on the mobile today. Listened, and it was from an employment agency, responding to an enquiry. Coincidence one: the name they referenced was the same as my younger brother's, though admittedly it's a common one. Coincidence two: said younger brother has a long, fairly inglorious history with employment agencies, as it took him quite some time to establish himself in his chosen career line, and he had a bad habit of cutting off and/or ignoring agencies when things didn't work out.
I therefore can't ignore the possibility he used my number when making an enquiry with one, but it seems very unlikely. To the best of my knowledge - we don't talk, unfortunately - he's been settled in his work for a long time, now, so would have no need to contact an employment agency. The chances of him getting hold of my number are remote, not least as I sorely doubt he'd ever even want to, and why he'd use it and his name is beyond me.
I'll talk with other family members, just to check, but this is most likely a coincidence. Quite a pronounced one, though!
ETA: Talked with family, and discovered he actually has his own business, so he may need to work with employment agencies, but that just makes him using my number all the more unlikely.