Can i freely call acros thousands of VoIP networds world-wide?

Is there any way to have free worldwide calls over VoIP acros multiple SIP servers?
I mean its probably possible two callers can call for free within one free VoIP provider, but i mean i have an free account on SIP server, i meet on the internet someone from other end of the world and he also have SIP (with different company, paid or free), and i will be able to call him for free..
How is that possible in general?

PS: some free VoIP providers (not sure if they offer what im asking): <- this service may allow calling between 2,000 VoIP networks for free

PS2: i have linux server and i found free SIP software:
curious if it can be used to make such world wide free calls what is needed to bridge calls across all VoIP networks for free

Thank you