Yeah, you need to go to Dollet to get the Siren card. It's also the key location involved in the Card Queen Quest.

There's also not one, but two Timber Maniac magazines to be found, as well as a humourous little demonstration of the Galbadian Army's ineptitude if you get past the soldier guarding the entrance to the Communications Tower.

And @ Merk, there's nothing in the Communcations Tower anyway except a Blind draw point (I think). The only monsters to fight are Anacondours and Geezards, which can be easily encountered in a lot of other places. So I guess Square didn't see much point in giving us access back into it.

Oh, and the music in Dollet is nice.