When the user tries to make a Call from the call log, the call is directly done using MozTelephony API. This means that if any other way to dial-in a phone number is installed on the device, the user will not be offered to choose that alternative. This affects, for instance, the Loop client. In order to make Loop more discoverable, we suggest to use an Activity in this case so the user can choose which one from the applications capable to dial a number he wants to use. The suggested approach here is making the Call Log App uses the Dial Activity. The problem with that approach is that current Dial Activity, requires the user to click on the "Dial Button" for security reasons. In order to avoid the user clicking on the "Dial Button" once on the dialpad for this particular case, we suggest to use a HMAC validation mechanism in the Activity so that the Dialer can check if the request is coming from contacts, and in that case, dial the number automatically so the user does not need to click on the green button.

According to https://bugzilla.mozilla.org/show_bug.cgi?id=988279#c1 this won't be part of Loop integration.