Maybe when Joyce was alive and footing all of the bills, she, for some reason, didn't feel that her daughter should have a phone, then, after she died, Buffy either just never got around to getting one or didn't feel that she really needed one. From a practical sense, just imagine Buffy trying to sneak up on a cluster of demons, trying to hear what their sinister plans were for the next apocalypse, and suddenly Dawn calls her to remind her to pick up a gallon of low fat milk and a box of Oreos on the way home. Or, she's trying to sneak up on that same bunch of demons and the Double Meat Palace calls to tell her that the 3rd shift counter girl has the flu, and she has to come and fill in. Set it on vibrate?No good. Demons probably sense vibrations as asily as they hear sounds.
Or, maybe Joss just never had a reason for Buffy or any of her friends to need a cell phone.