There are some VOIP systems that work well on Ubuntu.
So why not use them "for" Ubuntu ?!
There is some "shopping around" to find ones that work well, and which services are either free or reliable.
Ubuntu is in a position to provided a trusted service to its users.
The telecomms and media industries are full of sources of additional revenue - advertising, partnerships, hardware re-selling, professional (pay for) upgrade packages, etc.
Ubuntu would benefit from being able to establish voice comms between project members both for Ubuntu OS and fro programs.
Users might be able to use it for their desktop VOIP systems (e.g. Ekiga). The service could be ready to run soon after OS installation.
Old PCs could get re-used as email, browser, and VOIP terminals.
Ubuntu enters marketplace with opportunities to make "some" profit, promote the brand, showcase capabilities and user environment.
A separate or subsidiary entity could be created to run the operation.
A server network could be set up around the world. Local users could voluntarily provide bandwidth and/or hardware to initiate a local service - removing expansion costs from Ubuntu.
Very easy to be an alternative to skype.
Local servers could link into the local phone system, though this service may need to generate revenue to cover costs.
Those users who have already set up VoIP servers, could link into the Ubuntu network, which would help the rapid expansion of coverage.
Some "exchanges" may want to limit what kind of users they permit, so some system of managing this would be handy. But in the first instance, the "exchanges" which are prepared to let anybody access, and others who have their own systems of access limiting could be readily added to the network.