Right, they sell something they don't want you to use. And, for what reason?

I bet they will get sued sooner or later. It is ******** to limit what kind of device a user is using. FUPs are also ********, and so is 'X times the average user uses' because they are vague contracts. Just state the user is allowed to burn X GB, or unlimited. Simple and clear.

If they heavily filter they abuse their power, possibly net neutrality, and perhaps even unfair competition. In any case you can then use SSH/VPN/IPsec + SIP or HTTPS + SIP (yes, this exists).

If you use T-Mobile NL's unlimited data plan you can e.g. use 10 GB and they'll contact you about it, but won't terminate the contract, even if you do this regularly (I know someone who does this).

I read 3G has a round-trip of 150 ms. Might be too much for some people.

EDIT: If you're a lot outside of the country which your 3G contract covers consider to buy a (temporary) subscription to e.g. iPASS. This has the potention to lower your variable fees a lot. Especially useful if you're self-employed and travel a lot.