They must be marking that up severely
Try getting a new sim, and asking to move that number to it. It'll fail, but they can give you the network it's on. If prompted just give an excuse that you are doing this for an elderly neighbour and they wrote their old number down wrong.

Once you have the network, you can contact them about geolocation services 'my name is Anthony Fakename, I lost my phone can you help?'
( willenium Fist bump for verification, Wed 21 Apr 2021, 17:31, archived)

And then they ask you a security question to verify you're the owner of the sim
hardly going to geolocate some random stranger for you are they
(dribble, Wed 21 Apr 2021, 17:40, archived)

unless the security question is "Are you a builder named Tony?"

( Turing's Artificial Hand Sig Sauer, Wed 21 Apr 2021, 18:00, archived)

"What was your first pet's owner's name"

(dribble, Wed 21 Apr 2021, 18:04, archived)