Cards are useless and expensive, I have a adapter - from a company called ACN- this replaces my phone line, as with this adapter you have to plug your internet ethernet cable inside it-as it runs through it.
Basically it runs like a sim contract, you chose your minutes allowance for UK use - it gives u an option to add international minutes for each month--for Pakistan it was an extra £3 for I believe 500minutes, the normal landine to ring UK was £7 per month.
Best thing with this adapter is that u can add more of less minutes every month - direct debit set up.