Apple watch

long story short is that i got an iphone off my gfs cousin and she didnt have the password so i had to long reset her gmail and her icloud so i could put in password to remover her icloud from phone. Ever since logging in anytime i try to download a game occasionally or when i try to link my apple watch it wants me to verify her old phone number wich neither i nor she knows.. and the phone wont let me link up even if i log into a new icloud on settings please help