If the add ons screen fails to load (you just see loading), it's probably related to a caching issue that iOS4 has. Fortunately, there is an easy fix. Close Acrobits Softphone or Groundwire (close it completely, don't just move it to the background). Then open it again and you should be able to see the G.729 Annex A codec when you go back to add ons.
If you have a jailbroken iPhone (or other iOS device), you may also want to check out the following article.