To give a little history here, in the old days we had the GPO later called BT who owned the entire phone network from the wires in the ground to the exchanges and so on. When your phone didn't work who you gonna call? BT.

Since then things have changed

Firstly in an effort to look like the government was doing something to promote competition, OpenReach (That's Can't Reach to those of us in the business) was formed to be independent of BT, which its not and never will be. OpenReach brought a whole new level of poor service to the market and continue today to be the number one cause of faults on the network.

Digital Came

So a long time ago SIP was created as a way to connect disparate telephone switches together in a way that worked over IP networks. Its fairly week, has high overheads and brings a whole world of complexity to the job but once setup it works most of the time. I suppose I should take a moment to mention IAX2 which provides encryption, compression, and reliability to the market but its uptake has been slow.

With SIP came along SIP Carriers. Companies who's business model was carrying SIP traffic (calls) from one place to another. This is mainly because the market puts prohibitive costs in the way of companies connecting to BT (IPStream) directly so there's a market created where none should be simply because of that.

Complexity and Routing

So we now have the BT PSTN Network, the original network where analogue phones are connected and System X which routes the calls. Then we have BT's IPStream where network operators can connect and route calls to and from BT networks. Then we have these 'SIP Carriers' that connect companies to the BT and other networks.

Finally we have the resellers, yet another layer

Since SIP Carriers tend to have a minimun spend and traffic and smaller businesses can't meed these we have the resellers who buy traffic from the SIP Carriers and then sell that onto small/SME businesses and leverage the market for profit.

Let's talk about the market and profit for a second. BT Charge an Ingress and Egress charge of something like 0.6p per call, nothing after that. Billing per minute went years ago so the only charge is 0.6p. The SIP Carriers are paying this charge and therefore they are charging their calls out to the resellers at something like 1p a call.

The resellers will charge this out at 0.05p per second or 3p per minute to the companies using their services and therefore, if they have lots of LONG calls they are quids in, lots of short calls their out of pocket. If a reseller has several carriers on tap then they can move traffic between them to route the longer calling customers on to the cheaper pence per call, and the shorter calling customers onto the pence per minute routes.

Who's Fault is it?

Everyone get's failed calls, more so these days with so many companies involved in carrying the call, but when it fails who do you ask? Well, the simple answer is no one because your going to waste your day, a day that you'll never get back talking to people who don't care and don't know.

Let's take a call from a mobile phone to a SIP small business...

Mobile Network over GPRS
to
Mobile Operators Switch Network over the Internet
to
BT's PSTN over IPStream
to
SIP Carriers interconnect to BT over IPStream or other.
to
SIP Reseller
to
Destination

Any one of those points can be a point of failure, and a large part of that transit is over the internet which is well known for its reliability or lack thereof.

I hope this helps some people understand the market we're working in and why its just not possible sometimes to say for sure why a call failed, where it failed and who's fault it is.
E&OE - Prices approximately correct at time of writing