You have to make sure the phone you buy is compatible with your dads carrier. Find out what your providers network runs on, GSM or CDMA. Sprint and All-Tell are off the top of my head CDMA networks. The others use GSM.
The other thing to make sure of is that you purchase an unlocked phone. Phones that are unlocked will work with any carrier (as long as the phone supports the standard they use, ie GSM or CDMA). This way if you change carriers you wont have to buy a new phone. If you want the latest and greatest phones, you should purchase them from someone online, in the UK. They get all the cool phones first (after japan that is).
I've been drooling over this phone and will promptly order it the second its released:
http://www.infosyncworld.com/news/n/5944.html
The Nokia n91: 4gig harddrive, PDA capabilites, MP3 playback that pauses with incoming calls, and Wi-Fi!! It will seamlessly roam into publicly accessible wireless networks (ie for normal computers) and use VOIP (free phone calls whever your on a public wireless network!!!) ! Sorry im a geek. Only problem is its like 800 bux. But thats not bad considering its like 10 devices in one.