embedding sites inside other site, in iFrames, is one of those things that browsers sort-of support, but don't like.
In IE's case, it's considered a "security" issue (because it allows one site to track users using cookies etc).
At the end of the day there are settings in the browser, and different behaviors from one browser to another, but you can expect to see support for iframes from different sites get worse not better as the browsers tighten up on privacy, and as people care more about privacy etc.
In the long run your best bet is to create separate pages - let the user go from one site to the other (and vice versa) but try and avoid iFrames when you can.
Of course regular research via google is also recommended - this isn't a NetTalk issue, it's a web/browser issue, and from time to time things change - browsers get settings to turn stuff on and off and so on.
cheers
Bruce