How to bid on AdWords calls (CPP)?

The AdWords call extensions option to add toll-free phone number in your ad is really helping Search3w clients to achieve one buck conversion.
Is it?
Well, it was like that until we noticed sporadic dropped calls on the Google extension phone number and a drop in conversion.

The reason is that competitors are doing the same, so you have to bid higher on the calls.

Google explains

Max. CPP (cost-per-phone call) is the most you’re willing to pay for a call to your ad-specific forwarding number.

How it works: Using call extensions, you can show a special toll-free phone number in your ad. When users call that number, they’ll get forwarded to your regular business number. This feature is only available in the US and UK, and you must meet a minimum threshold of calls.

What to bid: CPP bids must be at least $1.00. The higher your bid, the greater your chance of ranking higher among other ads.

Manually dialed phone calls

The number of phone calls manually dialed to the Google forwarding number displayed on your ad when it was shown on desktop computers, laptop devices, and tablets over the date range you’re viewing.

But where exactly can you bid on the phone cost?

Take a look at attached screenshot to better understand how to do it.

<1> Select “Ad Groups”
<2> Segment your view by using “Click Type” option
<3> You can clearly see the dialed phone calls for desktop and mobile devices
<4> Click on the desired ad group row and change the bid in the “Max CPP” box.
One of my rules of thumb is to divide Max CPA by 4.
Saying that, if Max CPA = $90, the Max CPP would be $22.50