Suddenly can’t call or send messages but can receive them

So I was messaging my grandfather when he sent me a text and I tried to reply. It wasn’t going through so I tried to call him and it wasn’t going through. It went straight to voicemail so I thought his phone might’ve died. I went back to the house. And his phone wasn’t dead, which was weird so I tried texting and calling my sister and it worked. He tried to receive calls and text from my sister and it worked so I’m not sure what’s going on. I can receive messages and calls from him. But I cannot send messages or call him and this is only happening with him because I can call and text my other contacts just fine and same with him.