Ok, first off, loading Yahoo takes about 900 kilobytes (kB), not kilobits (kb).
Your 10minute conversation at 15kbps works out to 9 Mb, which is only 1.125MB.
And in reality cell phone codecs only take even close to 15kbps when they're running at full quality (and cell carriers being what they are, it's my understanding that they almost always skimp on that quality by at least half).
Wikipedia says AMR (the codec used in GSM and UMTS) varies between 12.2kbps and 1.8kbps.
Even the full 12.2kbps works out to 915KB for a 10minute conversation, the 1.8kbps rate only uses 135KB.
Of course, I think those are single channel rates, and you'll normally send as well as receive and thus double the data transmitted.
Overall I wouldn't call the voice call's relative data size minuscule, but it could easily wind up being less than a large-ish page load requires.
But in this case, it might be more appropriate to compare bandwidth needs. And in that measure the voice call really could be minuscule in comparison, since it's load is spread out over minutes instead of seconds.