Most prominent of these and available in Thailand are the offerings from Nokia based on Symbian S60 (e.g. E-Series 61) and that based on Windows Mobile 6 (e.g. HP's iPAQ 512).
Note : Though the HTC-Touch is based on Windows Mobile 6, it does not have the SIP support included, I heard it was due to software license restrictions.
You can connect to the Internet via Wi-Fi (FREE or otherwise) and use that connection to connect to a VoIP service provider. Imagine sitting in a Starbuck's and using your mobile phone to make calls but not paying anything to your mobile operator ! And it appears that True and Nokia have tied up to offer free Wi-Fi to N-Series Nokia models available in various places !
How does it work ? Well, these phones support SIP, which is a VoIP protocol that is fast becoming the standard and its supported by many programs/devices (except Skype). With built-in SIP support, just add an account from a VoIP Service Providers and you now have a portable IP Phone !
Simply (well a number of steps are involved to configure your phone) sign up for an account with a VoIP Service Provider and then configure the settings on your phone. Thai Telephone provides some setup instructions to connect if you have an account. Once you are all setup, when typing a number to call, you will have the option of making an Internet call or a GSM call.
Restrictions apply of course and here's a bunch :
1. Connecting to Wi-Fi networks. You might need an account with a Wi-Fi provider if you are out of the office. In your home or office, you could simply connect to your existing Wi-Fi networks.
2. Firewalls and Blocking. Some Wi-Fi networks may block ports required for SIP in their firewalls, so even though you are connected you might not be able to place calls.
3. Quality. Just as in VoIP requires a good broadband connection, your call quality could degrade if the Wi-Fi network you are connected to is heavily-used.
I am waiting for the day when we have a outdoors Metro Wi-Fi network - multiple Wi-Fi hotspots in town that lets your "roam" from one hotspot to another and keep you connected, then you will have a real Mobile IP Phone ! If you are in Singapore, the island-nation already has a FREE wireless broadband ... how envious !