While the native messaging of PureOS will be through Matrix, it will also support SMS. SMS by default is incredibly insecure and is a privacy nightmare. Some privacy can be restored by encrypting messages in transit between phones. There is an Android app called Silence (a fork of the popular instant messaging app Signal) that does just that.
Implement Silence compatible encryption in the default PureOS SMS app thus allowing users to send encrypted SMS messages both to other PureOS users and to Android users who are using Silence.
For more information see this github issue: https://github.com/SilenceIM/Silence/issues/670