sip trunk to asterisk
If the call is terminating on the 2800 ( ie ip to tdm) there is no need to transcode….
It would just be a standard sip offer/answer dialog.
As long as one codec matches it would be fine.
If you have 2 ip legs you should look into cisco border element configs as an SBC.
You can then decide how to work the codec selection and how the cisco interacts( interferes may be a better term) with the media session between the 2 legs Transcoding would only be necessary if you have two IP legs and each leg needs to use a different codec.
Leg 1 g711a and leg2 g729 for example.
The 2800 can do this