I'd like to think that part of the problem is that the usual operating systems that are installed on Raspberry Pis lack the power management that something like Maemo has. This is a software problem and is fixable.

Also, the Raspberry Pi Zero which was suggested, draws significantly less power than the traditional model B series. I'd personally like to see a phone built from the Pi Compute Module (draws a bit more power than the Zero but not close to model B). It's easier to embed than the Zero and can potentially be upgraded. A new Compute Module based on the Pi 3 is due in a few months.

There's a lot going for the Raspberry Pi and it's community is huge compared to ours. I think a Pi phone has potential to be successful if done right. I bet the Android Pi community would love to have a Pi phone.