I tested a few options to get the button to action within a Tour but..
You can use these in a html page:<a href="tel:0400-1234-5678"></a>
<a href="tel:[0400-1234-5678]"><span class="bottom"></a>
...but I added an id to try to trigger a CallToAction JS function on a hotspot but it doesn't work... <action name="onclickspotpoint11">js(onClick=callToAction(callme));<a href="tel:0400-1234-5678"><a id="callme"></a>
<a href="tel:[0400-1234-5678]"><span class="bottom"></span><a id="callme"></a>
... and <a id="callme" a href="tel:0400-1234-5678"></a>
Even if nothing worked, it should still open a box asking what application to use..
You are only going to be able to do this using a device with phone options.. Not sure if you can do it using a Mac or PC with some kind of software to act like a telephone..
You might have to wait until Monday for Gerald the guru on all this coding stuff... I just blindly fiddly until stuff works..
There is a new Plugin 'PanoTag' but that just a social media thing.. It would have been handy for that to have this feature.. Perhaps ask for it as a Feature Request.. emmm Just another one to fall into the dark void of lost suggestions... Never to see the light of day...