Sayid, will you EVER know how long the battery will last? In season 6, just once, I want to hear him say 'we have a full battery, take your time calling'.

If you like "Lost" then you like Science-Fiction. If your perception of time is different from that of another locale, then the exchange of information would also be effected. For instance, there would be a significant delay in the calls to the island. Furthermore, this could also effect causality. If time on the island moves faster (relative to external locality) then any calls from the island would be received before they were sent. Obviously, this is impossible. I believe this is why there are side effects. The particular heading used to leave the island could be explained using an Einstein-Rosen bridge (wormhole). However, this still doesn't explain how Jack & Sayid could have spoken to each other via a satphone. Although, in an earlier episode, Sayid does mention the advanced technology of the phones being unlike anything he has ever seen. I would suggest that perhaps there is a captured submicroscopic black-hole within the phone, utilizing the Schwarzschild radius as a conduit for information. I'm a fan of Sci-Fi, I never knew "Lost" was considered part of the genre until I saw it listed as such here on Hulu.