One of the pay as you go options worth considering is Lyca Mobile. Some pay as you go services give you no historical record of the calls, others will let you see details of the last dozens calls or so. With Lyca, after you have registered online, you can log in and see (and print out) a record of the calls for the last 3 months (number, date, time, duration & cost). There may well be other services with a similar facility. It's worth checking if such a printout would be acceptable to your employer but I suspect that, when Lyca charge just 5p per minute for calls to landlines, they would be delighted.
Lyca sim cards can be bought for £2.99 from larger Asda stores, 99p from corner shops or for less than the cost of a stamp on eBay. They can also be obtained online "free" when you buy a £5 credit. These sim cards will work in any phone that is either unlocked or (I think) locked to Orange. Most phones from CarPhoneWarehouse will be unlocked.
www.lycamobile.co.uk