Incoming computerized calls from scammers causing our line to lock and squeal.

For the last couple of months we have had various computerized calls from google maps/listing and other scams call our number and cause issues.  Because they are now spoofing local numbers that could be from a possible customer, we have answered the call only to have an extremely loud squeal on the line.  When we attempted to hang up, the caller will tie up our line for several minutes.  Any call that may come in for the afterwards is a struggle to hear the other person because the squeal cuts in and out.  We are losing business.  

We have tried using ATT’s lovely repair line but, needless to say, this issue is not one of their choices nor can it be detected through their check on the phone.  We’ve tried to talk to a live person.....unless we are buying, there’s no body home.

As much as we pay for a business line and we can not block incoming telemarketers, is there anyway to stop this harassment and prevent losing our customers due to a lack of protection from our phone company?