I find people's cell numbers all the time online. Often very minimal sleuthing involved if you have their name, or are connected via social media, or have other basic info.
(Second includes a lot of info about the usual reverse lookups when you gave the number and no name, but also some about this situation, the reverse)
Use These Strategies to Find Someone's Phone Number Online
5 Best Ways to Find or Lookup a Cell Phone Number Online
Thanks. I never knew all that and I bet no one I know well does either. I'm not a big SM user though except for WS.
I don't know if GP's family was into sleuthing cell numbers when they were frantic. If I were in that position I'd probably use what I had. Texting wrong numbers happens all the time, of course. One of my relatives even had an issue with someone who claimed "but we sent you a text!" Yeah, to her landline.
JMO