We use a tool that records any calls that come in from your campaign. Then, we manually score every call to ensure we get the best quality data.
Most agencies will count a call that lasts 45 seconds or longer, for example, as a conversion. But we try to minimize false leads by hiring a designated team to listens to the calls that come in from an ad and give it a score. Our call rater team also gives us the added bonus of helping us to uncover any issues that might be negatively affecting conversion rates during the calls (like an automated answering machine that is confusing people, for example, or low quality leads who are asking for something different than what your business sells, etc.).