You have probably seen the URI tel:
However, here's a trick to dial in to a conference call that requires you to punch in a conference id, indicate whether or not you are a leader, and then pres # afterwards.
where 8885551212 is the phone number, 12345 is the meeting access code.
This string will join you as an anonymous participant.
Try creating one and emailing it to yourself on your smartphone. It works on almost all modern phones.