https://whispersystems.org/ TextSecure / RedPhone and/or
https://silentcircle.com/ are a good place to start -- but AFAIK might not provide all the protection you need in a case like this.
Assumptions:
* the government has (or can easily) link you/your associates to a SIM/Router/DSL/Phoneline
* link your ip to said sim/router
* the government probably already have you listed as an "enemy" (assuming you've ever said or done anything useful in terms of protest, aren't completely spineless or apathetic)
Historically (in Latin/South America), at least one group will have support from the US -- so assume all VPN traffic is logged for trying to match up things like twitter posts to in-country IPs.
The only reasonable action is to use TOR for onion routing, and encryption tunnelled through that. This will still show that you communicate covertly (covertly, being "not-open"). It will still show bursts of activity, and clustering of devices (Note that if you both have access to location meta-data, and have half a brain and a couple of informers/officers noting down where people are gathering, you can feed that back into lists of SIM/IMEIs to target...).
The best approach would probably be to generate a rate-limited stream of traffic over TOR, and allow that stream to be replaced by communication as needed. I don't know of any tool to do this for phones -- but layering TOR and jabber+OTR might be about as good as you're going to get for now?