For basic/mostly voice services using quad-band GSM, I would look at Sony-Erisson (Z750i from AT&T, 3G-unlocked & debranded) and Nokia as these tend to be easier to unlock and most importantly, reflashed to generic firmwares vs. those AT&T or T-Mobile branded interfaces. Motorola L7 (quad-band) is a candybar and uses a usb charger, which can be charged via the cable - it's been around for a while and easily cooked with a different/newer firmware yourself or at a local shop inexpensively, plenty of eBay & priced around/under $100 unlocked (as low as $50 lately) - and OEM batteries (800+ maH) are inexpensive & long lasting.

We just came back oversea earlier this month and our LG CF360 (AT&T Gophone, 3G-unlocked QB, horrible bloatware) and Nokia 3555 (T-Mobile Prepaid, unlocked QB) also worked fine with local GSM sims in Hong Kong, China & Vietnam - which has roaming partnership with AT&T and T-Mobile here. (Paid practically nothing for these 2 during special promo period)

You might want to consider getting a dual Sim, unlocked quadband phone from these makers as you will be able to standby for both your US-based GSM carrier like AT&T while using your oversea/foreign prepaid sim on line 2, without carrying a 2nd. phone - especially if you are interested in voice services only. These are a bit pricer between $100 & $200 on eBay from oversea seller, stick with major OEM makers & avoid the "clones" for easier tech. support & simpler programming (thus, stay away from Windows, Android or other newer OS)

Another places to check is on cellhut . com as they do have specials from time-to-time, both regular unlocked quad-band & dual sim versions from LG, Samsung, Nokia & others.