Received mini frame before first full video frame

Dear all,

i have configured two Asterisk PBXs and i have connected two video phones (Yealink VP530).
When i try to call, the call ends when it appears the below message:
chan_iax2.c:11673 socket_process helper: Received mini frame before first full video frame

What does it mean?
How can i solve it?

Thanks and regards,
Davide