On the one hand, between Doug's engineering skills and all the junk he carries between worlds, he can presumably kludge together a 1992 cell phone with some more modern parts to make something much more compact. On the other hand, being seen carrying around a tiny cell phone would raise a lot of questions among Usagi's peers, so carrying around a 1992 brick phone may still be the way to go. Maybe give her a discreet Bluetooth headset so the custom cell phone could remain at the bottom of a purse?
“I really hope I’m behind this convoluted mess; at least that way I’ll be able to get revenge by doing this to myself. I won’t even have to feel bad because it’ll be all my fault.” - Harry Potter, The Master of Death by Ryuugi.