Hello & Welcome to Just Answer,
I'm Greg and will do my best to assist you today.
Sorry I didn't see your question earlier and to hear of any issue you are having with your phones.
From the symptoms you mentioned, it does SOUND LIKE the issue is with the provider's equipment or service.
You should be able to CONFIRM that by connecting a "basic phone" to the phone line - in place of the MAIN BASE of the Panasonic system - and TESTING if you have the same symptoms.
What I mean by "basic phone" is a SIMPLE CORDED PHONE - that do not require any external power (but may have batteries). You can find the at almost any store that carries phones, or even a hardware or drug store, for typically $15 or less.
I have a link below (to Amazon) that shows the types of "basic phone" I'm referring to.
Click here --> http://amzn.to/2i55fi7
I also have links below to the electronic (PDF) version of the manual for this system.
Please let me know what you find through your TESTING, or if you have any other questions about your phone system.
If this has helped answer your question(s), please take a moment to rate MY service level (5-star is my goal) so that I get credit for my time helping you with this question.
You should see some "stars" (and SUBMIT button) to do that, but please let me know if you do NOT see them (you may have to REFRESH your screen) or can not use them.
Thanks, ***** *****