bocastephen

Simple answer - localphone.com

lowest rates, easiest setup and the largest number of features including a callback service that works in many countries.

My google voice number is my main line and when I travel abroad I forward it to my localphone incoming number ($1/mo) and calls are automatically routed to whatever SIM card number I'm using anywhere in the world.

To make outbound calls from abroad cheaply, use the callback service - you dial the callback number for the country/nearest country you're in, and they will call your number back (your foreign SIM number needs to be pre-registered, but that's easy) and from there you dial anywhere in the world at localphone rates - this is priceless for those of us who have unlimited incoming calls on our foreign SIMs.