See here:
http://www.prepaidgsm.net/forum/euro...ip-client.html
note however that there is "doesn't block" and "allows"
Vodafone zorgeloos internet works with voip, but it isn't allowed.
Vodafone used to have "internet totaal" which allowed the use of voip. But it seems like it doesn't exist anymore. Pretty sure it was post-paid only.
As far as I know you can voip with the t-mobile post-paid internet options, but their website isn't that clear about it. So you should ask them.
the answer to the question about using it in the train depends really on the route you are traveling, but not sure if voip will work very well anywhere. Because the speed will go down if you are moving. 3g coverage will be prety good in the western part of the country, but less so in the rest of the country.
And even the gsm network won't work without interuptions, for example in tunnels at Schiphol, Rijswijk, Rotterdam Blaak, etc,
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