Is possible Free VoIP acros the worldwide VoIP networks / SIP servers ?

[Fli]

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):
https://www.linphone.org/free-sip-service.html
https://www.getonsip.com/
https://mdns.sipthor.net/register_sip_account.phtml
http://www.voip-info.org/wiki/view/Free+VoIP+Networks (http://www.voip-info.org/wiki/view/Free+VoIP+Networks)
http://www.sipbroker.com (http://www.sipbroker.com) <- this service may allow calling between 2,000 VoIP networks for free

PS2: i have linux server and i found free SIP software:
http://www.kamailio.org/w/
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

[RealFan]

SIP on the iPhone is already in the form of a bunch of free applications. With the release of the new version of iOS, it will be possible to keep the SIP application in the background. One drawback on a modern iPhone during an SIP / Skype conversation may be an incoming GSM call and your conversation will end. Therefore, I use provider like that (https://hottelecom.biz/sip-numbers.html), where these problems are all resolved and a separate phone number is provided. The price and communication quality of this service is much better than that of the free versions, and the price for the service itself is simply ridiculous.

[MaryPPerillo]

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