QKSMS, available on fdroid and Google Play, is the gold standard for SMS. It is free and open source:
For sending SMS using Android I heavily recommend QKSMS over the Google/Android defaults.(https://github.com/moezbhatti/qksms)
I'm sure there are other OSS alternatives but this one just works for me.
This project, Traccar SMS Gateway: https://github.com/traccar/traccar-sms-gateway
It's based on QKSMS: https://github.com/moezbhatti/qksms
QKSMS makes use of smsmms: https://github.com/klinker41/android-smsmms
I love GPL and FOSS :D
It's not. Google stopped open sourcing a lot of their apps with Android L. IIRC the material Calendar app also isn't open source.
Source: I was working on https://github.com/moezbhatti/qksms when Messenger was released.