I use Vonage, a voice over IP service that allows me to have a US phone number while I'm here in Japan. I have personally only used Vonage with the DSL service on base, and I'm getting mixed reports from people who have tried it with the wireless service; some say it works, others say it doesn't or that it's very frustrating. With the base DSL, the signal drops sometimes due to old wiring in the building, so the call drops, but most days it works. If you live off base, you shouldn't have any problems, since they have fiber optic internet off base in Iwakuni.
You can choose from a list of area codes, so you can have a phone number that's local for your friends and family (subject to what Vonage offers, of course), even if you're far away. You can take it with you when you move, and always have the same telephone number, which is great for us military folks!
You get all the same features you would through your regular telephone company (call waiting, caller ID, voicemail, *69 call return, call blocking, etc) and it's all included for a flat fee of $24.99 a month for the unlimited calling plan. There are other plans as well, such as unlimited international calling.
Vonage offers a special deal through the Refer-A-Friend program, where you get your first two months of service free, and I get credit for two free months as well, which is why I'm posting this.
If you decide to sign up for Vonage, please think of me and help save us both a little money: just use the referral link (http://www.vonage.com/friend/) and enter my Vonage telephone number (859-514-0586) when you sign up. There is also a box to enter my number on the main Vonage.com home page.
Below is a copy of my contact cards that I received when I signed up with Vonage. See...it's not a gimmick, it's just plain good common sense to save a buck! :)
*And by the way, if you decide to call me on my Vonage phone, please be cognizent of the time difference. If you're not sure, visit my Time Zones page to double-check what time it is here. I don't like 2am phone calls, and usually set the Do Not Disturb feature before I go to bed and disable it to receive phone calls when I wake up, but I'm human...I forget sometimes!