TELECOM Digest OnLine - Sorted: Re: Advice on One Call in SS7 Versus Two Calls in VoIP

Re: Advice on One Call in SS7 Versus Two Calls in VoIP

Thor Lancelot Simon (
Fri, 25 Feb 2005 20:44:10 UTC

In article <>,
<> wrote:

> The issue that I'm running into is this. In VoIP, this looks like 2
> calls to the service providers that we're talking with. They want to
> charge us for the DIDs, the inbound call, and the outbound call. I
> know in SS7/ISUP you could do a call forward, and this would remain 1
> call -- not 2 calls.

"in SS7/ISUP" you'd still end up creating a third leg, tying up twice
as many trunks: so the telco would still charge you for the extra
call, because they'd get double the cost for the single shot of
revenue otherwise.

With actual SS7 interconnection and the ability to set AIN 0.2
triggers on your subscribers' phone lines, you could implement this so
that the voice path were unwound all the way back to the originating
end office switch and rebuilt, so logically there would be no "third
leg". However, good luck trying to actually do this if you are not
administratively part of the same telco that owns the switches to
which your POTS subscribers are physically connected. You *will not*
be able to get the required TCAP interconnection and trigger access
you would need to implement this service using SS7 under an
interconnection agreement, and even if you were able to, you would
probably end up paying quite a lot of money for it per call even if
you avoided making that third leg in the call path physical.

In other words, services like this are *not* economical in the
"SS7/ISUP" world unless you happen to own the telco to which the
subscribers are actually (not through resale) connected. Believe me,
I spent five years pushing AIN interconnection to try to make a
service that sounds much like that of your client economically viable.
It didn't work.

Maybe you can get a VoIP provider to go for it, since they don't have
physical call legs to worry about so they don't *actually* incur twice
the cost, but claiming that you wouldn't pay for two calls in the SS7
world is simply wrong and won't be a useful analogy unless the VoIP
provider you're dealing with is, to put it bluntly, rather stupid.

Thor Lancelot Simon

"The inconsistency is startling, though admittedly, if consistency is to be
abandoned or transcended, there is no problem." - Noam Chomsky

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