[Arcten]

I've done quite a few phone screens with coding over the past few years, and the best solution I've found is a headset.

Speakerphone means that the interviewer hears every key-stroke and likely can't hear you. A decent set of earbuds with a microphone can be had very cheaply, and to me are an absolute necessity for a phone interview.

I now use them even when not coding, so my hands are free to take notes on the call.