Lost audio when answering a call between two T54W phones

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We've been deploying the Yealink T54W's lately and have been receiving a lot of complaints that when a call is answered the start of the call is not heard.

We have been able to reproduce this in a lab environment using 2 x T54W and doing the following:

• Call from T54W to another T54W answering and saying 1, 2, 3, 4.
• On each occasion we hear 2 or above, never 1
  

Doing the above, we are using G729, PCMA, PCMU codecs.

If we remove G729, PCMA, PCMU and only have G722 then you will always hear 1.

We have also tested the same scenario between two T42S phones and always 1 is heard so no dropped audio.

The other test we did was to go from a T54W to a T42S and 1 is also heard, so it only appears to be an issue between two T54W, or at least this is the most reproducible combination.