While skype is not ideal for discussing math, and it would be much preferrable to sit side-by-side, looking over the same sheet of paper, it is an option if your schedule doesn't allow for a common time slot to see me in the office. In particular if you are a working or otherwise non-traditional student subject to strict scheduling limitations.
For advising purposes, I have found skype to be almost equally efficient as in-office meetings. Only be aware that I may wish to refer to my own hardcopy notices, which I have in my office.
I am an evening person, so 10 or 11 pm is an acceptable time for skyping, but 8am is reserved for emergencies :-) Saturday, and Sunday afternoon, are eligible, even though I may not always be able to accommodate them, but Sunday morning I'm usually at church.
If we are both online at the same time (by coincidence or by pre-planning) and have both running the skype program, you can basically give me a ring from skype to skype as with a traditional phone. This connection is free (except for the internet service or data plan that is being used to make it work). It allows to turn on a web-cam, which comes in handy when showing a hand-written formula by holding into the camera. Otherwise I advise turning the camera off to save bandwidth. There are two caveats with this method: (1) One side may well have the computer running but not actually be next to it, and therefore not hear the ring. (2) Depending on privacy settings, we couldn't call each other, unless we are in each other's contact list. So we'd need to set this up beforehand if you'd like to use this method. (See below)
The computer-to-phone method incurs costs similar to a prepaid LD phone card and works simply by one party (for our purposes: me) having an account with skype and asking skype to dial any desired phone number, domestic or international, landline or cell. (Skype warns that it is not designed for 911 calls). You can use skype to make free calls to 1-800 numbers. A phone number needs to be attached to the calling skype account to show up as caller-ID to the called party. Skype verifies the validity of this phone number. Of course with this method, the camera feature is not available.
In any case, I'd prefer to have such skype-to-skype meetings pre-scheduled rather than by just ringing me up, simply because this is more reliable (provided both our internet connections are reliable).
If you do not have skype installed, you may request for me to call you at a phone number that you may give me. Normally, the cost for doing this is sufficiently low that I don't mind it. But you should be aware that my skype account is attached to an overseas cell-phone number of mine, so don't be surprised when you expect a call from me and see a foreign caller-ID. Also if you ever choose to call me under this cell phone number, make sure that I *am* actually overseas, b/c else that cell phone will be off. (And calling European cell phone numbers costs an arm and a leg, since the caller, not the recipient, pays airtime charges, with European cell phones being recognizable by having their own `mobile' area codes. So no, you don't really want to call me at that number... But if I *am* overseas, you can use that number to send me a text msg at an affordable cost depending on your carrier.)