One of the first cloud voice services was audio conferencing. Audio and web conferencing are a big component of UC&C (unified comm and collaboration). I am as guilty as the next agent in not pushing enough of this product.
There are a lot of providers, including quite a few free ones. Apparently, politicians like using the free ones:
"During the 2012 election, both Presidential campaigns racked up millions of FreeConferenceCall.com minutes -- President Obama's campaign used over 1,000 FreeConferenceCall.com accounts and millions of conferencing minutes. Governor Romney's campaign also had multiple accounts and totaled considerable minutes of conference calls, although far less than the Obama campaign. This was the second consecutive national election where both presidential campaigns used FreeConferenceCall.com."