Video preview Grandstream GDS3710 and GXV3240

We have a freepbx version
We connect two Grandstream devices a video door GDS3710 and a videophone GXV3240.
We make a call from GDS to GXV but there is no preview video.
If we answer the call the video works correctly.
We have checked the progressinband parameter but it does not work.
Could you help us with this issue?
Thanks in advance.

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