I recently did this and followed these instructions that I found on another iphone blog:
In order to use this simple hack, your iPhone must be jailbroken (see quickpwn tutorials) and you must install Mobile Substrate, OpenSSH and VoIPover3G (reboot after install) from Cydia.
Assuming you?ve installed these apps and that you know how to SSH into your iPhone, follow these instructions:
1. SSH into your iPhone and go to the root directory ?/?
2. Navigate to /Library/MobileSubstrate/DynamicLibraries
3. With a text editor, open VoIPover3G.plist
4. Look for:
Filter = {Bundles = (?com.Fringland.Fring?, ?com.apple.AppStore?, ?com.audiofile.Interstate?, ?com.apple.MobileStore?);};
Now add this part: , ?com.slingmedia.SlingPlayer?
It should look like this:
Filter = {Bundles = (?com.Fringland.Fring?, ?com.apple.AppStore?, ?com.audiofile.Interstate?, ?com.apple.MobileStore?, ?com.slingmedia.SlingPlayer?);};
Don?t forget the ?,? and the quotation marks!
5. Save the changes you just made and upload the file back to its location: /Library/MobileSubstrate/DynamicLibraries
6. Reboot your iPhone
Thanks Dan for the tip
7. Done! SlingPlayer should now work over 3G!
In order to edit the VoIPover3G.plist file using SSH from a windows pc, I did a google search for a program called "editssh". It was a free download and did the job.
Good luck