Does anyone have any experience with using callback services when travelling abroad. I'm going to Europe and India and need to call the U.S. without spending a fortune. Thanks.

Depending on how long and exactly where you are, it may be easier just to rent a cell phone while you are there. If you hvae one here, most of the carriers rent them at very good rates for use there.

Based on the advice that I read on these boards, I signed up for Telcan (www.telcan.net). I have since used some of their other services, and plan to use their callback feature for some international travel I have scheduled for the end of this year. If you do a search for "telcan", you will find a fair number of positive comments.

This has been discussed in several other posts but I'm too lazy to point you at them...so, a couple quick points: you may want to get an unlocked GSM phone and use it in Europe/India. If you're on T-Mobile or Cingular, you can get your existing phone unlocked if it isn't, assuming it has the right bands (you'll need 900 and 1800, though you may be able to squeak by with just one of the two depending on where you are.) Otherwise, unlocked GSM phones are quite cheap on eBay.

Prepaid SIM card are ridiculously cheap in India. Airtel now charges something around 0.5-2c/min for calls in India and about 7.5c/min for calls to the US. Other providers are similar (I recommend Airtel because when I was there a few months ago, they were the only one that let you buy recharges when you were out of your home area.) At those rates, unless you're on the phone for hours a day it's so much easier to just use a prepaid SIM. Europe is a good bit more expensive; I'm headed to Europe for a couple months myself in a few weeks so I'm trying to figure out the cheapest option there... Callback might indeed be the way to go for high volume.

I've been doing it with Asterisk (specifically Trixbox) for quite a while.
I also buy a $30 sim card for where ever I am...then I call a local number (from www.voxbone.com) and it rings me back. Then I have a dialtone just like I was sitting in my house in Virginia.
Its also nice for people who want to reach me....they can call my home and press "5", or what ever I setup, and then get transfered to my cell where ever I am in the world.

The least expensive way to call back is just to use an IP phone -- as far as the system can tell you're still in your house. You'll need to be near a network connection, such as wi-fi, but you can do that almost anywhere nowadays.

There's a entry about IP Phones on the Road on the road in my blog.

I have Asterisk as well. However, with Voxbone, there is a setup and monthly fee. I find it is usually cheaper to use a prepaid MCI Card from Costco, and use the local SIM to call the MCI access number, or call from a payphone to the toll-free MCI access number. Rates back to the US are as low as 10 cents per minute (e.g., from the Netherlands). Also, using Voxbone, you still have to pay for the local call to the number you have purchased from Voxbone.