See, that depends on what kind of average speeds you'd be able to get. Why not try running the speedtest on this site to see what your connection is capable of and then post back.
However, in the meantime, if you'd like to try a VoIP application, then I can recommend Skype as it's the best I've seen. Head over to
Skype - internet calls - download Skype free now - free calls - official website and download a copy. It's completely free and trustworthy. It should work on even the slower broadband connections (as long as it's not retardedly slow), as it will detect the amount of bandwidth available and use a codec that is suitable. Use it to make free calls from one Skype user to another. If you want to call a non-Skype user on a normal landline or mobile, anywhere in the world, then you should purchase some SkypeOut credit, which will go a long way compared to credit in your mobile.
See rates and purchase options:
Cheap calls to landlines and mobiles with SkypeOut
The only problem is that you're subjected to Luxembourgian VAT when you do purchase credit.
Hope this all helps.
Seifer.