As a long time business traveler can I just share one recommendation for low cost calling.
Look up TALK360, this Netherlands based service gives great calling rates to and from anywhere in the world.
I use it to make all my calls to clients as it's by the far the most cost effective service I have found.
As long as you have a decent internet connection the call quality is superior to any other service I have tried. (I've tried them all!).
Very easy to use with a good app.
Check it out for yourself.
In addition to what talk360 charges you, you will have to pay (to giffgaff, or to whatever network you are on) for the data that is used by the app.
On giffgaff, if you have a goodybag you probably wouldn't notice the data the app uses because it will come from the goodybag. But if you have only non-goodybag credit, you will be paying giffgaff for that data at 5p/MB (or substantially more if you're calling from outside EEA), in addition to whatever you're paying talk360.