@rl_dane @spaceraser you could do some really cool things if the software were actually open though. Like having a firewall for phone calls and messages. You can sorta do this today but it almost always involves giving some third party app access to your communications metadata, which I am not okay with.
Not totally sure what you mean by "firewall" in this context, but there are APIs for anti-spam services in Android, and there are FOSS anti-spam apps (which honestly don't do much, because last I checked, there's no central service to collect those bad phone numbers).
SMS & MMS messaging in Android is totally open, and you can get any number of apps to handle your messaging, or even filter them, IIRC. #RCS is completely closed* and worthless, though. Absolute waste of paper.
* To any possible reply guys, yes, the spec is open. But there is no FOSS implementation, and even if there was, the RCS infrastructure is completely closed. fugghedaboudit.