The Behavior of “Clean” and “Dirty” Call Tracking Numbers and its Impact on Call Advertising


In our personal lives, an unwanted call such as a “wrong number” or “auto-dialer” is a nuisance. For businesses, such a call wastes time (and money).

As digital advertising campaigns increasingly use call tracking numbers (CTNs) to measure phone calls, minimizing unwanted calls is important for two reasons. First, an unwanted call may misrepresent the value of one’s digital call advertising campaigns by inflating the number of “true calls” one has received. Second, servicing the unwanted call distracts from servicing current or potential customers.


We obtained 1,750 new U.S. and Canadian local and toll-free numbers from several carriers, and assigned them to a dedicated account without publishing the numbers to monitor the activity of unwanted calls they would receive.

We also studied the historical call activity of more than 500,000 CTNs in use or available for assignment by Marchex Call Analytics covering more than 10 million phone calls.
We observed that:

  • About 15% of CTNs are “dirty”, that is, they receive materially more calls than the average. The remaining CTNs are relatively clean, receiving, on average, less than about 1 call per month. (We say “call density”, which is about 0.025 calls per CTN day.)
  • The call density for each CTN does not change over time; in particular, it does not decline as we wait longer. Dirty CTNs stay dirty, clean CTNs stay clean.
  • Although the preceding is true as a general pattern, abrupt changes in behavior do occur for isolated CTNs. (We observed a small number of CTNs that had a low call density have a short burst of with significantly higher call density.)
  • Four of the CTNs that were part of this study had previously been owned by Marchex, but had been returned to the carrier (presumably because they were dirty). These CTNs were found, once again, to be dirty.
  • In most cases, a majority of the “noise” can be identified as spam callers, vendors or solicitors.

We conclude that the prevailing industry opinion that CTNs receive fewer unwanted calls over time does not hold true.  CTNs will continue to receive unwanted calls, and once they are published their background noise will increase as numbers are added to spam lists and solicitor lists.

What does this mean for marketers?

First, it means that marketers must understand that not all calls are created equal.  In the click world, we are all used to using tools to identify repeat clicks or to measure “bounce rate”.  Equivalent analysis must be done on calls to truly understand how many “revenue-potential” calls are truly being generated by a campaign.

Second, marketers with particular sensitivity towards spam or vendor calls should maintain a blacklist or work with a call tracking or advertising company that will maintain one on your behalf.  (Marchex has such a service included in its analytics and performance-based products.)
If you’d like to discuss how this applies to your business, please reach out to the Marchex Institute at 1-800-840-1012 or e-mail us at

John Busby
VP, Marchex Institute

John Busby leads the Marchex Institute, a research and analytics team that publishes findings on mobile advertising and the growing digital call advertising industry. The Marchex Institute also provides custom research and consulting services for customers on their mobile and call-ready advertising campaigns. Previously, Mr. Busby served as Vice President, Product Engineering at Marchex. You can find him on Google+ and Twitter.

Posted in Data, Marchex Institute, Performance Advertising, Technology

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