does the phone have to have a serial interface jack on the outside?
For the moment, yeah: partly because it provides assurance that the serial interface feature is available (e.g., not disabled in firmware so they can use those CPU pins for a camera interface or somesuch), and partly because I don't have the equipment and skills to hack the super-fine-pitch SMT boards you find in cellphones (This is one of those cases where age and treachery does not overcome youth and enthusiasm :().
Even at dollar-store prices, money is too tight to buy a bunch of phones without high confidence that they'll work out. And it's hard to find which phones are even made with that chipset, much less whether they support the serial interface.