This week I saw that O2 in the UK are offering a call plan with "500 free minutes from your favourite place", which is an example of what's called agile billing, or in other words a billing system not just based on time and distance, but based on some pattern or behaviour of the customer.

I don't know if O2 use Dialogic products to provide this service, but the fact is that they could. Quite often these products are based on SS7 monitoring technology: that is, a new application server is added to the network that passively monitors the signalling links (either SS7 or Sigtran), and the information gathered is then processed and fed into the billing system. In O2's case, they probably just pick up the cell id and the IMSI (unique user id) so that they know who and where you are before they process outgoing calls. So if you're in your "favourite place" you get free minutes, otherwise they're billed according to the normal rules.

Telcos like these "passive monitor" solutions because they don't have to be fixed into the signalling path, and so are not a point of failure. It's also easier for them to work with 3rd party companies offering such services, since all they need to do is copy the signalling traffic out to the app server (perhaps using an Ethernet switch with a port mirror function), and all the "magic" happens elsewhere, with a very small capital outlay.