This is <a href="http://en.wikipedia.org/wiki/Carrier_preselect">defined in Wikipedia</a> anyway in some PBX you may have e.g. 5 lines and each of these are with different providers (Example provider1 is good for international mobiles, provider2 for landline calls etc). The CPS needs to be done normally by the operator to call a given number by using the most convenient provider as an example to call US you'd dial 1033#001800; in some PBX this can be preconfigured so the users won't need to remember all CPS codes.

This is <a href="http://en.wikipedia.org/wiki/Carrier_preselect">defined in Wikipedia</a> anyway in some PBX you may have e.g. 5 lines and each of these are with different providers (Example provider1 is good for international mobiles, provider2 for landline calls etc). The CPS needs to be done normally by the operator to call a given number by using the most convenient provider as an example to call US you'd dial 1033#001800; in some PBX this can be preconfigured so the users won't need to remember all CPS codes.