Fix: Take care of the logic for Dialing

Fix: Take care of the logic for Dialing

The logic for dialing a call while other calls are in the system is a
bit more complicated than what was implemented.

There are several cases to consider:

- Active Call and Held Call
	In this situation we cannot dial, the user must drop one of the
	calls.
- Active Call and Waiting Call -> We have to put the active call on
  hold, unfortunately there is no standard command to do so without
  affecting the waiting call.  If the modem has a vendor specific
  implementation, then try to use that.

- Active Call only -> Put the active call on hold.  We use the
  swap_without_accept implementation (if exists) to be safe, otherwise
  we fall back to hold active calls.

- No Active calls -> Simply dial