I've had the autonomous call happen only once. A former paper girl called the teenage boy in the household and asked him out. I never even thought that was possible, and in the many times I've had teens go to prom since I've never seen it again.
I always have found it interesting just how much Prom can change relationship dynamics. In one of my current games I have a house full of teenagers (twin boys and twin girls, with the girls both having IFs). One of the girls went to prom with her imaginary friend (they were RI at the time) and came out going steady. The other four (three siblings and an IF) went solo. One of the brothers wound up with a babysitter as his RI, the second brother wound up with his other sister's imaginary friend as an RI, and the last sister wound up getting into several fights, with no RI at all.
I have to wonder if the chances of finding a romantic interest during prom are trait-driven. The sibling who came out of the dance without a romantic interest has the Rebellious trait.