I like both the Sugoi Midzero Zap and Subzero Zap because they've got the most ergonomic shaping so they stay up the best (the least amount of crotch droop). I think the coldest I've run in the Subzeros is @15F and they were fine. If my butt gets cold I wear either UA compression shorts or just Hanes Shaper undies (you know, basically the control top portion of pantyhose) because I just need a very light extra layer for warmth and they don't have annoying seams. Or I add a pair of very light wind pants from Zoot. They don't have back pockets but I wouldn't put a phone back there anyway. Even at 15F my lower back gets far too sweaty for electronics to survive.
Frends know gud humors when dey is hear it. ~ Da Crockydiles of ZZE.