Home calls ringing on mobile phone

I am a Comcast/xfinity Triple-Play customer.......TV/Internet/Phone.  My mobile phone account is with AT&T.  Whenever I receive an incoming call on my home LAN line, my cell phone also rings.  The call is not being forwarded.......it is just ringing on both lines.  Why?  How do I eliminate this?  The only way I have been able to stop this is to delete the xfinity connect app from my cell phone.  Thanks.