On Sun, 18 Jul 2004, Bill Stewart wrote:
If you're trying to build a usable cellphone, you've got much more stringent design criteria than a deskphone.
I am painfully aware of it.
You've got packaging requirements that force you into serious industrial design if you want something pocket-sized with good battery life, plus you've got to implement all the cellular interface features.
Or use the off-the-shelf modules for industrial applications that already do it, and add some glue logic.
If you're willing to build a backpack-phone, that's a lot simpler, because you can use a laptop with a [pick-your-favorite-cellular-data-standard] card and either a wired headset or a Bluetooth frob for a BT headset.
Check the Gumstix and the Enfora Enabler specs. The first is the equivalent of a grossly stripped-down laptop (80x20x6 mm, few mA sleep, 50 mA command-wait, 250mA full power w/o Bluetooth), the second one is the equivalent of a comm card (GSM/GPRS, 50x30x3 mm, tri-band 5mA standby). The laptop approach is good for prototyping, though.
I'm not aware of any cellular data cards in PDA-usable format (unless you've got a PDA big enough for PCMCIA), but you could take a GSM etc. phone with a wired interface to a PDA.
I'd try the Enfora module in that case. RS232 for data and control, and analog I/O for voice. The PDA approach definitely has its merit.