Their big idea is phone numbers that pipe all calls AND SMS into XMPP. It's $5 mo for unlimited messages, and the rate for voice calls is less than 1ยข/min (I forget exactly).
So now calls and sms for my second number come through an XMPP app on my phone (Cheogram), and through apps on other devices/operating systems (e.g. Snikket on iOS, Gajim on Windows and Linux).
Voicemail show up as attachments via XMPP in Cheogram, and it does transcription.
You can use pretty much any XMPP app you want, but I've found only Cheogram supports phone calls well. I do use Monacles Chat on my phone for messaging and disable message notifications on Cheogram.