This is always a challenge.
Rental phones and pay-per-call phones purchased stateside can be notoriously expensive, but are the easiest to procure.
"Local" phones are far cheaper per minute but can be a pain to procure in the US.
The best option, overall, is to have a GSM-compatible phone that is "unlocked" (e.g. you can swap any providers GSM SIM card in and out) that you purchase here in the states. Then, you purchase a "local" GSM SIM card, with a "local" number, which you slide into the phone when you arrive abroad. I've done this with varying success; Orange was the best provider I ever dealt with when doing this.
That said, I'm debating trying a VoIP solution and roaming from hot-spot to hot-spot. My cell phone bills when I travel border on the insane and I am trying to do things "cheaper" this year.
Cheers,
-Andrew
I'd add to Andrew's advice:
Buy a cheap unlocked cellphone. I've done OK on Ebay, but there are a number of other places on the web to get one (starting with ubid, overstock.com, newegg etc. etc.).
Buy a local prepaid SIM card in country of arrival. Orange, Virgin, etc. serve a number of countries: check rates before purchase. Insert SIM card, and go. Heck, at Heathrow and Gatwick they sell 'em in vending machines right outside Customs.
Another alternative is to get a prepaid international SIM card that allows free inbound calling. IIRC, you can get one at
http://www.telestial.com/. Then use SKYPE or another cheap VoIP service to call her via the internet. Much, much cheaper than roaming a US phone number over there. This is probably the best solution if she'll be traveling to more than one country. She can call you for a fairly low rate ($0.29 - $0.99 per minute, depending on the plan) and you call her back using your cheap service at a free inbound rate. I seriously looked at this when I went to Japan recently but decided I wouldn't be there long enough to get my money back vs TMo's international roaming.
Telestial has cheap international phones, too, so it can be a one-stop shop.
At a fixed location, if she'll have a laptop & aa broadband connection, Skype is really the best solution. Free calls computer-computer and a very low rate/minute back to the US. I can more than cover the daily hotel internet charge if I use Skype to call back to the US instead of a cellphone (but I don't do it for business calls as there is no encryption).