Change codec priority for internal calls only

[mk1518]

I have the global_codec_prefs set to PCMU,PCMA,G722. But I want to change this order to put G722 first for all phone calls between internal extensions only. At the same time, I do not want this to potentially mess up transferred calls: so external caller A calls internal extension B using PCMU, then B transfers to internal extension C (or merge calls), in this case the call should remain on PCMU not G722.

What's the correct syntax to add to dialplans to achieve this?

Thanks!