I'm not sure about the security protocols for Skype, but be careful with VOIP (voice over IP) phone services. They're not secure conversations, unless it's encrypted on both ends. So be careful what you talk about with your clients (i.e credit cards, addresses, personal details, etc.) on those services. Do an internet search for security and VOIP and you'll see what I mean.
Problem with this is that probably 98% of all phone calls are voip (even those that start on your end on copper). It's the only way phone companies work nowadays.
On top of that, copper lines are VERY easy to eavesdrop in on. You would have probably less security on copper than on VOIP. Fortunately, VOIP can be made secure, if you and your client want it to be.
Dan