Assuming you were able to compile the SDK and the linphone XCode project, here is what you need to do:

1. Include G729_preference in the Audio.plist file. You can either type all the info yourself or simply copy the G722 line and simply replace 722 with 729. Make sure you set Default Value to YES
2. In Project -> Target -> Build Settings find "Preprocessor macros" and include HAVE_G729
3. Run the project. The SDP should now include G729 in the invites or 200 OKs.

Note that G729 has royalties (aka patent license) so be aware. If you plan to deploy commercially, you will need to setup an agreement with Sipro.