I think it's in the T&Cs that VOIP services are disallowed.
I agree that this is unfair... people expect an internet connection to be an internet connection, and not crippled in any way. But the fact is that the Vodafone Broadband is not suitable for VOIP,, due to technical problems.. (pings are too slow, i.e connection isn't responsive enough,.. this also renders it unsuitable for online gaming)
Have you got your T&Cs?.. if not I'd suggest you get them immediately and read them.
Vodafone are obliged to provide certain info in their contracts... if they have not done so then they are breaking the law, and you should have a right to exit the contract with no penalty. See COMREG 07/49, and COMREG 10/34 for more info... COMREG 10/34, specifially tests the legality of Vodafones contract and has 'identified a number of areas for improvement'.. this is an euphenism for the contract being illegally formed. The 10/34 report was produced on 21April, 2010, so it's quite recent.
In the report COMREG also say that they expect the necessary changes to have been made by 1-June-2010... this date has now passed, and has there been any change?.. Well, I wouldn't expect so.
It's a fact, as published by COMREG, that all broadband operators in this country have illegally formed contracts, and have had them for the last eight years or so. Perhaps you could also write to Eamonn Ryan, the minister with responsibility.
I'd suggest you write to COMREG and ask them if you can exit your contract early, with no penalty, as the contract is illegally formed.