What phone do you have? If you have an Android, you can easily connect any USB keyboard to it using an OTG adapter.
It's Android, but discontinued--NEC Terrain. I read about otg on it before, but I don't think it supports it.
![Sad :(](https://cdn.geekhack.org/Smileys/solosmileys/sad.gif)
It already has a built-in keyboard, but when I have to text a lot of numbers it's a bit of a pain. Hence why I'm on the lookout.