I assumed P2P, since Joeboy mentioned "no problem, just go to this URL...". If you're not doing this in a P2P way, then you need an account with third party and the third party sometimes needs to transfer both your signalling and your audio. That means they need to get money for that from somewhere...
In that case you're pretty much in the same position as with Skype - just without a download. Account, configuration, potential payments, etc. remain.
Edit: Actually now I see a solution - if only one person had an account, this would still be possible and he could start a call by sending some URL. So for the other person this is "just go to this URL". Unfortunately we still need a third party to handle the traffic unless you require the account holder to ensure their network is capable of acting as a server.
In that case you're pretty much in the same position as with Skype - just without a download. Account, configuration, potential payments, etc. remain.
Edit: Actually now I see a solution - if only one person had an account, this would still be possible and he could start a call by sending some URL. So for the other person this is "just go to this URL". Unfortunately we still need a third party to handle the traffic unless you require the account holder to ensure their network is capable of acting as a server.