I know that Beem is a new project, but I didnt saw this feature "request" before.
In the company I work for, all employees have XMPP voice service on mobile through 3rd party clients, and we use it everydays as PSTN replacement.
Also want with my friends interoperability using xmpp.
Im sure this feature will rock.
ps: Maybe in the future -> Video/voice call - vp8/rtp