SIP : Attended Transfer, Blind Transfer & Conference not working properly

Hi,

I’m trying to transfer call to another phone both are Grandstream DP750 on the same Grandstream DP720.

Blind Transfer does not work at all. No ring.

Attended transfer & Conference have the same behavior … It works perfectly, until the person who transferred the call or initiated the conference drops out of the call. At this point, there is no more audio between the person who called and to whom the call was transferred. The call stops after 20-30 seconds.

I bought two phones to be able to replicate the problem at home and validate.

My account : 5217

Phone number for testing : 819-414-1982 & 819-414-1987

I've also open a case with Grandstream.

Thanks for your help,
Nicolas