Quite often when I call "800" numbers from the UK on my mobile, I get "please redial using "+1 8xx" because the "call isn't free". From landlines in the UK you often get a recording first, saying that it's not free, and to hang up if you don't want to pay international call rates.

To avoid all this hassle (and have one number in my mobile which works regardless) I always ask for an alternative to the "800" number.