Trouble with one phone number

From my landline I can dial anyone’s phone number and get through to them. There is one particular number that I dial and I get a busy signal. That person has checked all of their call blocking settings, etc., and they are clear. Why would I not be able to get through to one particular number? I have reset the modem, done all the Xfinity unplugged, replug, etc.  I have disconnected my phone etc as well.