@salahmounir you can't speak to an advisor as there's no call centre. However there is the ask agent online help system https://giffgaff.com/support/ask and community help in this forum which we are providing.
It seems to me that you have been running PAYG using your airtime credit which is expensive if you are not a light user or use the internet a lot. What you should do is to convert your airtime credit into a goodybag which gives you a bundle of minutes, texts and data at favourable rates.
Please see here for more on the tariffs http://www.giff.ly/ggdeals
To convert airtime credit into a goodybag/gigabag please see this guide http://community.giffgaff.com/t5/Solve-your-questions/Can-I-buy-a-goodybag-gigabag-with-my-existing-...