Receiving busy signal on landline phone

 For the past several weeks my landline phone (703-631-8077) sometimes does not ring for an incoming call, and I discover this only when I pick up the phone and hear a "busy" signal. Rebooting my modem (which I can do with the Cox Connect app) always fixes the problem, but this is happening about twice a week now. I do NOT have my Voice Everywhere app set to "allow incoming calls to app," and I do not want to do so because of the disruption. The app does usually show voicemails, but it never shows "recent" calls. I use a workaround by going to phonetools.cox.com, and this allows me to see my accurate call history in addition to voicemails. In other words, I need to use three sources to do what I used to do with the Cox Connect app alone, and in addition I am never sure whether my landline phone is "busy" unless I check for a dial tone.