Cool Tools: Native Union has solved my problem with a series of handsets that allow you to take calls using a traditional handset attached to your cellphone. I picked up the MM02, a fairly basic corded model featuring a cradle, that I have really come to appreciate (they make a cordless Bluetooth version, but it is significantly more expensive).
The handset connects to the iPhone via the 3.5mm socket on the top of the phone, leaving you free to rest the iPhone in the charging cradle, audio device etc. The handset is reassuringly solid, with a pleasant, matte plastic feel to it, and the well built cradle sits happily on a desk. There is an answering button in the centre of the handset that makes it easy to pick up calls, but one downside is that there is no keypad on the handset, so although you can dial out you need to use the keypad on your mobile handset itself, and that can be a bit fiddly. If you’re flying a desk like me, you may find that you make your outgoing calls on your desk phone anyway.
Moshi Moshi Manual Cellphone Handset [Cool Tools]