Samsung - Number not valid

To whom it may concern I bought a cell phone on Friday 3 June 2022 and on Monday the 6 it keep on saying number not valid and I went to jet store and it only worked and again yesterday it repeat the same problem number not valid,and when i wake up to day it was working so today went to jet store to lodge another complaining and I told them the problem that keep on persisting they told me that because my phone today it working there's nothing they cam do about that ,they must see the problem for themselves I told them but you saw it the first time I came to lodge a complaint and it keeps on going so if I go back home and its happens again what must I do ,I plead for exchange but they didn't want to so I'm so stressed please help