iMessage not working for one person on iPhone

My dad has an iPad with cellular data, he is able to iMessage me if I use my iPad, but when I use my iPhone he does not get the message, I however get all iMessages from him on my iPhone and iPad, I just can’t send or reply on my iPhone, it’s so frustrating…we’ve signed out of iMessage on all devices, signed back in, checked that I can start and receive message from my phone number, turned all devices on and off..it’s beyond frustrating please help ! Thank you