Sometimes, if you are sending out billing reminder calls, Verizon has a new 'feature' where they could mark your phone number as "Possible Spam". Other carriers might do this as well based on their customer complaints.
There is a web site for Verizon to request that this be removed:
Which looks like this:
Select "My business or enterprise phone number(s)"
My feedback is:
My business makes legitimate calls and should not be marked as spam.
In the above form, you can ask your 1800 Notify support team for total calls per month, or look on you most recent billing backup statement.
Suggested language for the "Provide further feedback" box:
We are the phone system provider for __(company name)________ - and send out billing reminder calls. The clients of ___company name___ are obtaining prior approval from the patients to receive an automated call regarding a billing matter. So this is a lawful reminder message according to FCC and TCPA regulations. Our client __company name___ will also contact you directly. The calls are not 'harassing' in nature and follow the following guidelines. We announce who is calling, provide a toll free call back number, don't call more than 1x per month, and only during regular working hours. Please turn off the "possible spam' on this phone number.