I've been happy with bid-to-call. It's super cheap and some of the calls are good. Most are not potential customers, but that is true for clicks, too. That's part of doing business. Overall bid-to-call has been a good thing.
However....a couple of the AdGroups I had it on started getting a bunch of calls. These were low activity AdGroups, and there were most calls than clicks (normally the ratio is about a hundred clicks for every call). The calls were under 30 seconds, so Google does not charge us for them. I am not worried about the cost from Google. I am worried about the labor cost for my client who has to answer the phone. Too many of these low quality calls becomes a nuisance.
To remedy this, I paused the AdGroups that were generating these calls (at least, according to Google these AdGroups were generating a lot of calls.) After pausing, there were zero clicks. But the calls continued to come in! Google still reports under 30-second calls from these two AdGroups!
Is this a new form of spam or fraud? After worrying about fraudulent clicks for years am I now going to have to worry about fraudulent calls.
This happened to my client over a period of two months. You might have experienced a random spike or, in the case of my client, we actually spent 2 hours on the phone with Google because we also saw a spike in the percentage of clicks Google deemed invalid.
Try checking to make sure you are not using the same phone number in the other Ad groups.
Check to see a change in your organic traffic.
click fraud is much easier than actually getting fake people to call as that would be expensive to run. If in doubt, call your Google rep and confront them about the issue