People calling us on Cell phones at home getting busy signal

We do not use Xfinity Mobile which I know has major problems with Wifi calling on an Android phone, but have Sprint as our carrier.  We have Xfinity for our at home internet and Wifi connection.  We have Galaxy S9 phones.  I use Wifi calling at work because in my metal building LTE doesn't work well.  Starting last Sunday, our son gets a busy signal whenever he trys to call us at home using a land line phone with our phones set to Wifi Calling.  He can get through when he call us anywhere else with our phones set too Wifi calling  except at home on the Xfinity Wifi connection.  Any ideas as to what could have changed in the last week with Xfinity.  It was working up until this last Sunday.