Not very. Almost all the early Phoenix missiles are all but unusable because of the cracks that develop in the solid fuel rocket engine with age. Launch one off your wing and the chances are pretty good that it's going to explode - either shortly after launch, or right at launch, taking your wing off.
The missile guidance systems themselves need quite a bit of maintenance, I am told, so the chances of a working Phoenix coming from an Iranian aircraft is very small.
Hopefully their new pilots don't know ...