Most players here use Skype cause its usually already installed to call family and friends and doesnt require someone to pay for a server - same thing for raidcall. However if a server is available then Teamspeak, Ventrilo, or Mumble are superior for the role because it doesnt require you to maintain a list of contacts and managing a conference call when you have more than 4 players.
Ventrilo is easily installed, then you simply login using the Server address with any Username and youre already talking to any user thats connected to the server at the time. I would use Teamspeak if it had better volume controls but PSO is so loud that you have to install the quiet sounds patch or have it mute the game while talking. Ventrilo supports up to 800% voice amplification so you can leave your important PSO sounds on (music muted) and adjust your speaker volume to allow voice that clearly overlays sounds.
No reason not to use it unless you have technical difficulties, you can even download a Ventrilo APP for your phone and use that anytime.
If anyone needs technical support then I can help, been fixing computer problems for almost 20 years.
Join in the fun! Its easy, free, and upgrades your gaming experience.
EDIT: Sever is hosted in London, England.