tSIP: sip2sip.info

sip2sip.info seems to be great service for any testing purposes and also nice free VoIP operator in general. I remember trying few free (totally free) VoIP services about 10 years ago, but I think none of them worked well in typical scenario with client behind the NAT and/or with misconfigured public IP address. They were not handling media streams at all, working just as SIP proxy/registration server, which of course was understandable given they were operating with no cost for end user. As sip2sip.info handles NAT, it just works without any requirements for the client configuration.

sip2sip.info configuration

From my short testing:

In general: two sides of the call must be interoperable - using common codec(s) and (if needed) same way of sending text messages (either SIP SIMPLE or MSRP).

iptel.org seems to be another free and easy to use VoIP operator. For test call sip:music@iptel.org can be used (works even without creating/using account from this operator).

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