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Where do VoIP calls to the PSTN go?

Started by SeanTek, July 26, 2020, 11:30:24 PM

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SeanTek

It's 2020. Pretty much all of the PSTN is digital. So, when someone places a VoIP call through an ITSP, where do the bits actually go?

For example, when you place a call to a PSTN number through Callcentric, Callcentric first decides whether to handle the call internally (e.g., to another Callcentric user), pass it upstream to "the PSTN", or do something else. When Callcentric passes "it" "upstream", what exactly happens technically? And who (between the telco companies) pays for it?

Given that everything appears to be hell-bent on sticking with G.711u, can one reasonably infer in 2020 that the call stays unchanged as G.711u bits for the entire journey for non-cellular network phone numbers, and for cellular network phone numbers, that there may be one transcoding between G.711u and the mobile operator's preferred codec for the associated mobile device?

I am asking specific to North America, including inside the NANP. No need to discuss international toll calling.

drgeoff

They go to wherever the ITSP has an interconnect with the PSTN.

No different in principle to how ISPs connect to the public internet.

SeanTek

I understand that, but I am asking, what happens afterwards.

Who is the provider (or one of the providers) that is the next hop after the ITSP, and how does this provider accept all this traffic? Do the bits just get pointed to, or through, different UDP connections on the public Internet? Or does the ITSP have a different wire (which also uses SIP?) that goes to a different machine in a hosted data center, which goes...?

drgeoff

#3
Quote from: SeanTek on July 27, 2020, 06:09:33 AM
I understand that, but I am asking, what happens afterwards.

Who is the provider (or one of the providers) that is the next hop after the ITSP, and how does this provider accept all this traffic? Do the bits just get pointed to, or through, different UDP connections on the public Internet? Or does the ITSP have a different wire (which also uses SIP?) that goes to a different machine in a hosted data center, which goes...?
Time you learnt to google for yourself?

eg
https://www.channelpartnersonline.com/article/clecs-unlock-pstn-gateway-for-voip-resellers/
https://www.commsbusiness.co.uk/news/thus-launches-carrier-sip-interconnect-service/