What app can I use for messaging in Mexico

I'm going to Mexico next week for a few days & I know that Sprint charges.15cents receiving messages and.50cents sending but also using an app costs 4.00 everytime I use it.
Is there a messaging app I can use that won't charge me or being able to at least use twitter?!
I have an iPhone 4 & it's also jailbroken if there's any apps that can help me.

There are third party apps that you can use. I'm not sure if you have these Voip apps like:
skype
viber
tango

These are some apps that I could see that fits for your messaging experience while you're in Mexico

Enjoy you're trip!