When I give my cell number to someone new, I also try to get their number and save it on my cell right then. If for some reason, I can’t save their number right then, I let them know politely that when they call or text me the first time, they should leave me a voicemail OR mention in the text who they are…b/c I don’t pick up unrecognized #'s or respond to text message if I don’t know who they are. I’ve never had anyone not understand my “rule”.