Sounds like you want a two-way alphanumeric pager that uses SMS and existing Cellular networks. A quick Google search didn't turn up any existing, though.

AFAIK, the chipsets used in phones do voice rather easily, so you'd have to work out a voiceless chip.( IOW, it doesn't cost much more to do voice)

Single use might be useful to some (where the unit gets sent back to get "refreshed", I don't like the notion of geuinely disposable electronics especially when they can easy be reused, or in the name of ones convenience.)

The best I'd think is buy a phone for ~$100 and service charge cards. That way, you'd get your own phone and number,

100 years ago they called that WIRELESS TELEGRAPH, go to a museum and see it, then build your own. WESTERN UNION provided the service before phones and radio were invented. Today you can make one most easily out of a "phone terminal for the deaf" and a CB radio. Search for "RTTY", the service is free with a license in my area, and can do email, and can be heard around the world on shortwave radio as beeping sounds.

The text messaging only phone now exists with the Ogo. But it's still a monthly plan... It would be nice for some to be able to use this with a prepaid card. Then again, this is only $18 a month! T-Mobile has the Sidekick with a data only plan for $30 a month.