Actually AMR-FR-HR in transcoder changes to PCM , in MGW PCM is changed to AMR(Compressed) and again in destination MGW, AMR backs to PCM and then in Transoder changes to one of the Codecs.
Now if A call use AMR codec the transcoder of B should use AMR too.
TC (AMR+TFO)–>MGW(AMR)–>MGW2(AMR+TFO)–>TC(AMR)–>Radio
so as can been seen , quality will be increased in addition some TC can be saved as well. i think changing PCM to AMR needs more resources in comparison with (AMR+TFO) to AMR in MGW