From Bulbapedia, the community-driven Pokémon encyclopedia.
The Pokémon Net Center (Japanese: ポケモンネットワークセンター Pokémon Network Center) is a building in the town of One Island, and is only accessible in Pokémon FireRed and LeafGreen.
In addition to the standard features of a Pokémon Center, it also contains the Network Machine. When functional, this connects Kanto and Hoenn's PC systems. When it is first seen, it is not yet active, but after the player retrieves the Ruby and Sapphire from Mt. Ember and Chrono Island, respectively, it begins to function. Celio, a good friend of Bill, works inside.
The music in the Pokémon Net Center has a slightly higher pitch than that of a regular Pokémon Center and appears to be a remix of the one used for the Pokémon Communication Center in Goldenrod City on the Japanese version of Pokémon Crystal after the Mobile Adapter GB is connected at least once.
The symbol seen on the front of the building later reappears on the GTSDP and Global TerminalPtHGSS, possibly indicating some sort of connection.