Hi Netta, I don't think you can compare what you might charge with what a call center pays it's employees. I don't know what the situation is in Greece in terms of minimum wage, etc. but as a business owner you have to pay for your own health care, taxes, etc. (things ordinarily handled by an employer, at least to a degree) so with that in mind, I would suggest starting by finding out what an in-house administrative assistant is paid (on average) in your country and then start doing the calculations to determine how much you need to charge to cover all of your expenses and make a profit as a business owner.
You could also check the web for websites of VAs who offer the same services you do, to help you determine a range.
I hope that helps
There are some resources you might find helpfu