Found this which may help...
All,
I just recently got my S3 (T-Mobile) on opening day 21Jun12 however could not get my wifi to work properly at home. It would say "connected" on my S3 but no internet and wifi calling. In addition, when I tried to "enable" my wifi calling, I would get a REG99 error. Tech support narrowed this error to an address update on my profile (911 address info) which was updated but the problem still existed. I troubleshooted with my router and saw no problems on that end due to the fact that 8 other devices connected fine. After looking carefully at IP addresses, I noticed my S3 persistently tried to connect with an IP of 192.168.100.16 where all my other devices had an IP that started with 192.168.1.(xxx). Check to see if your S3 defaults to a conflicting IP or IP that your Router does not accept (My router accepted standard IPs from 192.168.1.1 to 192.168.1.254). Before you do the steps below, reference your Router Configuration and check your starting and ending IP addresses, Gateway IP, DNS and Alternate DNS.
Now for the fix:
If you're S3 is connected to your home wifi but no internet, wifi calling, etc.
1) Simply goto Settings
2) Click on Wifi to get to the your wi-fi networks list
3) Press AND HOLD on the connected network (The one that says connected) and a window will pop up
4) Select modify network config.
5) highlight advanced options
6) Change IP settings (dropdown) to static
7) Enter a new IP address that your router accepts (ex. 192.168.1.10). Like mentioned above, my router takes 1-256 for the last number set. Be careful to not select an IP address that's already being used. You can easily see what IPs are used in your router configuration setup (something like attached devices).
Make sure your gateway is similar to your router config (usually 192.168.1.1)
9) Network prefix length - I left this alone at 24
10) DNS 1 and DNS 2 can be also found in your Router Config setup. (DNS 2 is usually your alternate DNS).
11) Click save and voila (You may want to turn wifi on and off on your S3 to recycle)
After 3 tech support calls with no resolution and researching this for the last 2 weeks, I found the answer to my problems. I can understand how annoying this problem could be for some people out there so I decided to register with XDA and post this for you all.
Hope this works for some of you. Enjoy!