..yeah, if you spent about three seconds looking at config_caster.ini, you'd see that it's 7500. It's also UDP.
Technically, there is another one, though. The port testing feature uses 7501, which is why you can't set that port. You also cannot configure your local port to be higher than 10000. None of this matters for general usage, though. (Newer stock Caster has this fixed but I didn't port the changes over because, well, who cares?)