To your request, Inmarsat C, a satellite store and forward messaging system, provides 2 way messaging (up to 32K each way) as well as SMS service. That's the good news.
The bad news is they weigh much more than the +/- 8-10 ounce small satellite phones. The 'C' I'm familar with is about 3 Kg, and requires a 12V battery, and is typically placed on vehicles or other assets that need to be tracked.
Some of the sat phones have a low rate data / IP data capability, so that may get you where you want to go on a slightly different path.
There are some systems that do exactly what you want in a **very** lightweight, very small package, more good news. The bad news, you can't afford them and they're currently only available to the US Government. Why mention it? Because there's a trend for this type of technology to migrate to commercial technology in time. The time will come within 3 - 5 years where realtime multi-mode tracking and SMS anywhere are an affordable reality.
If I really wanted 2 way data anywhere, I'd go with one of the lightweight 8-10 oz data capable sat phones.