I can understand Joss' saying that cell phones would have made things much more convenient and therefore less dramatic within the storyline. Buffy's rocket launcher in Season 2 was a great gimmick for a one time shot, no pun intended, but too much use of rocket launchers, incendiary grenades, guns, even something as primitive as bows or crossbows for that matter, would take away from the action of the show. If Buffy could just stand back a hundred yards or so and blast the vampires out of existence, or even just pick up her cellphone and call for assistance whenever she needed it, we would have lost all of that great hand to hand action. Either that, or we would have just had a female version of Blade, slaughtering literally hundreds of vampires at a time.
On the other hand, within the context of the show, I would hazard a guess that the main reason that Buffy and her friends never moved into the Twentieth Century, weapons wise and communications wise, was Giles. Even though he was the one to give Buffy the beeper (or was it Joyce, and Buffy was just suggesting that he use it to call her? Whatever...) he clearly resisted modern technology in the form of computers and probably most everything else. Look at that old beat up Citeron (?) that he drove for so long, and how he complained about the automatic transmission in the red sports car that he eventually replaced it with.
Clearly, modern weapons would have worked on many if not all of the things that Buffy ran into during her nightly patrols, because Wesley used them quite effectively on "Angel." And it apparently wasn't Watchers' policy in general, because the wet team who came to get Faith were very well, if illegally. armed. Even in Season 8, when Simone initially asked why Slayers couldn't use guns, Andrew, Giles' apprentice at the time of "The Girl In Question," at least, could give no more valid reason than, in effect, "They just don't."
By the same token, there is no valid reason, beyond dramatic effect, that vampires don't wear some sort of armor to avoid stakes to the heart or carry and use firearms, other than that one time in the series premier when Darla used a pair of pistols (It just occurred to me. Darla had a pair of .38's. Okay, 9mm's, but that work as well). Surely some vampire or demon would have come up with the idea at some point that one bullet from a sniper rifle would end all of their Slayer problems. But if that had happened, the show would have been over before the first commercial break of the first episode.