Yes, u can use them.
Few countries , it wont work as Qtel may not have contract with the network provider in that country.
If u wanna make sure abt any country, just call up 111.
You can use GPRS too.
Whatever service you use from hala abroad, u will get charged double bcoz your using Qtel Via local Network Operator.
Even when someone calls from Qatar to ur hala when ur in Greece, the person too will get charged double.

Its better to buy a local Sim out there!