Typical clueless ASA
Whilst I'm not disagreeing with all the ASA's decisions in this case the one regarding the possibility of broadband causing a problem with regard to ISP fair use policies and the point regarding BT's line rental are complete crap.
With regards to fair use policies, if the ASA had in the first place dealt with ISPs unfair advertising their service as unmetred then customers would know that this could potentially be an issue. Whilst the ASA allows ISPs to lie through their teeth and sell their heavily metred services as unmetred then of course consumers may get confused when their VOIP setup causes them to end up throttled, again though how is this in any way Vonages fault? It's blatantly the ASA's horrific incompetence that's to blame here.
Vonage have pointed out that it's unfair that people have to pay line rental from BT rather than just broadband costs (of which BT get a share) if they want to use Vonage via VOIP as their phone provider instead of BT.
As a result I don't think Vonage's advert was unfair in this respect as it's not Vonages fault BT has such a monopoly that customers have to pay line rental but instead BT's fault with it's lobbying against Vonage's request to skip line rental for people not wanting BT's phone service but still wanting broadband - it's clearly BT at fault here!