Using standards based VoIP is one thing, Skype is another. It was correctly pointed out that lower level bandwidths work fine -- such as G729, G723 or iLBC for example. Many consider G711[u or a] to be carrier grade, but this requires more upstream bandwidth than the other CODECs to work properly.
So, yes, you can use 3 for VoIP, provided you use the right CODEC or the proprietory Skype software.
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