Google voice gives me a phone number for that service.
I configure that number to ring as many or as few other phone numbers as I desire (in my case: my cell phone, work desk phone, and home phone) with rules based on time of day (so the work phone only rings during the day on weekdays and the home phone only on weekends with the cell always ringing).
The Google voice number also supports SMS and MMS along with visual voicemails.
Google voice also has a spam filter so that most unwanted calls/text messages don’t ring/notify the phone but you can review the messages if you choose. You can also block a number so that the caller receives the “this number has been disconnected or is no longer in service” message.
The end result is a sandboxed phone number I can give out while keeping my personal number, personal/private. Also if I change numbers/have to use another phone temporarily I can add that number to GV and work as normal then remove that temporary number when I have access to my number again.
From what I’ve been able to glean it looks like both mumble and matrix are closer to P2P chat solutions than a way to interact with POTS and SMS. It does sound like there will be a bridge to connect Matrix to SMS it looks, to me, like it will be using my number not its own. Which is good for the primary client as I do want my personal number to be able to send/receive SMS/MMS but I also want to keep my sandboxed number as well.
Hopefully that clarifies some.