IIRC Grab has gone to an in-app communication system (at least as of early this year) so a (local) phone number isn't (as) necessary anymore as it once was. Looks like you do have to register/log in with Google/Apple/Facebook however. Don't remember if I DL'd/activated the app before I used it in Bangkok and in Malaysia where I did use it. Certainly did not get a Thai SIM card for the 2 rides to/from BKK given a ~24 hr stay. May have used Grab again in JHB and/from airport and near-ish by hotel a couple of week prior to BKK use. Just used airports' & hotels' wifi to arrange the rides.