iMessage not working with one person

My messages are sending to everyone else but one person, I am receiving their calls and texts but that one person isn’t receiving any of my calls or texts and I’m not blocked, I’ve already reset my phone, I’ve logged out of everything, but it still isn’t working. It sends me straight to voicemail, but that person says my calls or texts aren’t being received.

[Re-Titled by Moderator]