Using Direct Routing - 3 Way PSTN call

I have Direct Routing and Users have E5 license assigned which gives Audio conferencing but users can't make PSTN 3 Way call. 

when they try to add a 3rd person to the PSTN call they heard "unknown participants is now joining" and dropped and the call even didn't send to the SBC. 

I have [Case #:24935948] already opened for more than 2 weeks with no luck. 

hope anyone can help.