I worked at Argos at Christmas when they were launching it, here's some explaination:

When you call a geographic number for your local store, it will just ring the phones in the store and depending on how busy the store staff are it can take ages to be answered. If you call when the store is closed, your call is never answered. The new 0845 numbers ring the store, but if the call is not answered within a certain amount of time (about 20 seconds or so) it is answered by an automated service which can provide information about store opening times, stock availablilty and suchlike, which are the most common reasons for people calling the store. This means that the customer can often find out what they want without the long wait, and staff in store are able to spend more time serving customers face-to-face.

Because this automated service is provided at a single location for all stores nationwide, the 0845 numbers (0845 1657 XXX where XXX is the 3-digit store code for your local store, as listed in the back of the catalogue) are used, and the call is routed to your local store initially.

Calling the old (geographic) number for your store will still get you through, but it may be a long wait. If you want to know simple stuff like stock information or opening times, either look on the website (free!), call Ring and Reserve (unfortunately an 0870 number  - 0870 600 1010), or the 0845 number for your local store.