Flashing phone OS

Hi people, I wanted to know if it was possible to "flash" (correct me if i'm wrong) or install our own OS on some phone, I'm not really interested in smartphones since they are rather expensive for what i'd do with them: making and receiving calls.

I'd probably end up buying a dumb flip phone since they are small, simple and do not need to be recharged every 5 minutes (pardon my exaggeration). The problem is, most phones UI Sucks big time and cannot be customized, so I'd like to program my own little OS with the features I want with a nice UI that's not nausea inducing to have my very own phone.

If you have any knowledge about phone software that you'd like to share, please do so, it would be much appreciated. Thank you very much in advance.

Regards.

I think your best bet is going to be looking into budget Android phones.

Android has plenty of developer support and therefore has plenty of different custom ROMs you can install, depending on which phone you get.

My suggestion is to get a Nexus phone, a Galaxy Nexus or Nexus 4 can be had for reasonably cheap these days.