Can't change default for Calls
I can select the softphone that I'd like to use, in this case, iSoftphone but when I close out Preferences in Facetime and reopen it, it's back to Facetime. If I select iSoftphone for Default and then click on a number in Safari without closing preferences then sometimes it will dial from iSoftphone and others from Facetime.
MacBook Pro with Retina display, OS X El Capitan (10.11.3)