Ron:
In order to get the cheapest rates with many providers you need to buy their 'bundles', which include a certain number of minutes and/or a certain number of texts and/or a certain amount of internet data.
Getting your phone credit via 'bundles' can really see your calling costs fall. (For example, Vodafone normally charge 35p per minute for calls but buying a suitable bundle can see call charges fall closer to the 8p per minute I've quoted for Asda Mobile).
However, unlike normal calling credit, 'bundles' always expire at the end of 30 days, so you lose any unexpired credit from them at the end of that period.
Puzzled001 clearly doesn't want or need such 'bundles', so he should be looking for a provider that offers low calling charges using 'ordinary' credit. That's why I suggested Asda. (They heavily promote their own 'bundles' but there's no need to use them to get low calling rates).