There are several reasons why this might occur, however, the most common reason is that you are using an intermediate redirection technique to track traffic.  

For the Call Tracking algorithm to correctly attribute visitors it needs to see the first 'hit' after the initial click that drove the traffic to your site
for example, the basic flow should look like this:

Instead you may be using an intermediate redirection technique which would look like this:

In the above scenario, you may not have the Call Tracking JavaScript code running on the site '', but you have it running on ''
This means all that the Call Tracking Algorithm can see is the traffic being referred from to

To fix this, you'll need to ensure that the JavaScript tracking code is running on your subdomain, and setup in the Tracking Pool configuration.

To do this Go to the Website tab and select Allow tracking on multiple domains, then select Save.

Test again from another computer and a phone you haven't used to call in with before, and you should now see your traffic being attributed correctly.