MobileTechReview.com recently looked at the Enfora GSM/GPRS CF Card which can turn your PDA into a phone. The quad band (850/900/1800/1900MHz) card uses the GSM service. The card is also unlocked: Get a SIM from your carrier and you are set to go. Change carriers and all you have to do is change the SIM.
Reception may not measure up to some smart phones but the reviewers found it quite adequate nonetheless. The included software (for both a PocketPC PDA as well as Windows 2000 and Windows XP OS) also got a good review.
This might be a good solution for someone not ready to upgrade their PDA just yet and needing to make phone calls on a less frequent basis.