The question is : how did Motorola implemented the DTX ? Did they follow the ETSI recommendations ?
I don’t know all the details obviously, but I gave you your answer. If you want to investigate, have a look on the ETSI website, see how they implemented the VAD2 with the AMR codecs, start questioning them. Perhaps Motorola is not using VAD2 ? I don’t know.
Now it is up to you to work on your vendor 🙂 I can’t share all details on this forum and I wouldn’t want to start assuming things on Motorola or Alcatel systems publicly…