Thanks for your answer!
Actually, SipBroker (http://www.sipbroker.com) has several PSTN numbers in many parts of the world, that allow to access almost any voip provider from a traditional landline.
I've used that successfully with people calling from UK, Ireland, Argentina and Belgium directly into my VoIP FWD line.
I was wondering if such thing was available for landlines in Greece.
I've seen that there is something like that from evoice, but it does not look to work properly to act as gateway.
Regards,
Santiago