Sorry bbb_uk, I think that posts's a little confusing - because you've mixed up the terminology.
In layman's terms, having a CPS (
Carrier PreSelect) calls means that, although you keep your BT landline, pay 'line rental' to BT (BT Together Option 1 / the Unlimited Weekend Plan is the minimum) and your incoming calls still arrive on BT (so Caller Display, 1471 and 1571 work normally), you have opted to have the BT exchange programmed so that, by default, (nearly) all your outgoing calls are sent via the CPS provider (and BT don't even 'see' the calls even though they carry the first leg).
The 'default' part of that sentence is the key to CPS. It means that, if you want, you can use a prefix to send an individual call via another provider (i.e. override the CPS programming for a single call). For example, using just a 1280 prefix will send that call out via BT, using just a 18185 prefix (not 128018185) will send that call out via
www.18185.co.uk (an '
indirect access' or 'override' provider).
In other words, CPS allows you still to pick the best carrier for your calls even though you've opted, normally, to use one provider (e.g. Primus Saver Option 2 because of the free evening and weekend UK 01/02 calls).
There is, of course, another type of calls provider - those providing specific access numbers and making their money via the cost of the call to the access number. I call these
gateway providers but some call them dial-through providers.