I recently got a MobileWorld SIM card for only about £10 which includes that much worth of calls. It boasts cheap international calls in particular, and I got it from Carphone Warehouse with whom I guess Mobile World must be connected (judging by the Mobile World contact email address which is @cpwplc.com).
There's a fair manual in the pack, but it doesn't tell you what you need to know to get the voicemail service to work in the first place - only how to use voicemail!
To activate voicemail on a Mobile World SIM, you have to dial 222 from the phone. Then you'll get a text message, which you have to delete, turn the phone off, turn it on again. Only now will your voicemail be working.
You must also check your phone settings to ensure that it will divert to voicemail on no answer etc - I thought it was, certainly before I bought the MobileWorld SIM card, but it must have reset itself when I swapped SIMs, so I had to fix that setting too.
Hope that helps someone. Yes, you could get that info by calling their customer services on an 0800 freephone number, but if you are calling with a Mobile World SIM they charge you even for a freefone call, and it can be a long wait.
More info about Mobile World, pretty the same as what's in the manual, is on the Carphone Warehouse site (but not the 222 info I just gave).