Find someone with a US phone number. There are various services online that will provide you with such - Skype for instance. You only need it the first time you redeem a reward, I've never been asked again since
This doesn't work! Bing rewards detects it as a VoIP number, and it doesn't allow VoIP numbers.
Tried with Skype number. Didn't let me.
Tried with a Google Voice number (note: I ported my old T-mo number to Google Voice). Nope, my mistake! I shouldn't have ported my number to GV...
So even when you are overseas the only that will work is getting a prepaid SIM card from a US carrier like T-Mobile that does supports calling or messaging in the country you are traveling to. By example: GoPhone from AT&T (prepaid) is only supported in Mexico and Canada.
Tried T-Mobile and it worked like a wonder in Costa Rica. You can get prepaid SIM cards from any online store. Just be sure that the SIM can be activated online or you will have to call to the US.