If you make lots of calls each month then it might be worth looking at a SIM only plan that comes with an unlimited minutes bundle. Most of the phone networks now offer these and they allow you to call all UK landlines, mobiles and a selection of other numbers as often as you like for one low cost line rental each month.
More about these plans
Can I really use an unlimited amount of minutes?
On most SIM only plans yes. If a fair usage policy is in place this will be highlighted on the details page before you sign up. Its worth noting too that unless stated, these SIM plans are for personal customers only. The terms by most networks state that they could terminate a contract if it is proven that you have used one of these plans for business use.
What types of numbers are included?
All calls to UK landlines calls are included in your unlimited bundle as are those to any UK mobile phone that begins with 07. Sometimes calls to 07 numbers which are held by customers of places such as the channel islands are excluded.
SIM only deals which include unlimited calls are now available on plans with just a 30 day contract. 12 & 24 month contracts are also available and sometimes offer better value for money if you don’t mind signing up for a longer period of time.