Your usage of VOIP over their Flat-M tariff appears to be outside the terms of the tariff - so their curtailing of such usage could hardly be described as being: "T-Mobiles fault".

I use T-Mobile Mobile-Broadband-Plus (in the UK) which specifically prohibits VOIP usage - if you want VOIP then their Mobile-Broadband-Max tariff accommodates such usage.

Don't get me wrong - I think the Service providers are taking the p*ss - For example if an account is sold as "UNLIMITED" but there is a little * which says "Max 3GB a month Fair usage applies" then it is NOT UNLIMITED. Be that as it may, if I pay T-Mobile for a 3GB per month data pipe, then it should be up to me how I use that 3GB - browsing, email or VOIP.

Also - as you have found, if your VOIP traffic doesn't look like VOIP traffic because its inside a VPN pipe then you don't get penalised.