I'm in Virtual World L, and want to talk to Totally VIrtual. Through some mutual agreement about displaying presence (such as a Friend List), I can see that Totally Virtual is available.
Virtual Worlds are a communication medium. The key element is presence - knowing that people on my contact (Friends) list are available and I can contact them by text or voice. This would enable presence to be visible in both directions - seeing on my cell phone who has their phone on or is available in a virtual world, or from the virtual world, which Friends are available via text or voice channels. As with other services, this requires a way to find the Friend and for the Friend to agree to have presence visible to me.