Looking at a phone from an engineering point of view, what roles could we give to it?
* a complete radio, with antenna, modulator/demodulator, amplifier and whatnot,
* it is battery-powered, which suggests everything here is low-power
* user I/O: speaker, microphone, screen, input device, at least if it is to be operated as a standalone device and not a peripheral
* SIM card I/O
* user I/O (screen, speaker, microphone, buttons) if it is to be operated stand-alone
* firmware gluing all of that together
For test and measurement, some want features like ability to drive the device from a computer. Why did phones even skipped that? Maybe avoiding to make SMS spam too easy? I still want it.

In addition to the useful mobile internet tethering (USB, WiFi), and ability to use an external keyboard:
What about a RPC interface for sending SMS or starting calls from a computer keyboard?
Such a device could be added on the lab bench along with the other, to give the lab operator (you, me) ability to place and pick-up phone calls and exchange SMSes.