Constant Busy Signal

I have been to the Apple experts...no joy. iPhone X - when I call my husbands phone I get a constant busy signal. Have tried: airplane toggle, do not disturb is not on, my number is not blocked, reset network, erased and restored from backup. I call call everyone else, others can call him, he can call me. Text still works both ways. Apple experts say both phones are fine. Updates are current.