In the dialplan I am connecting the caller via Dial(SIP/XXXX,30,Tt) to an extension. If I do that with the Flags “Tt” the caller voice is not beeing transmitted! If I do the same thing without the flags it works like a charm. What could be the reason for this? I need to allow the call to be transferred.
Is Asterisk behind NAT? Do you have direct media enabled? What version of Asterisk?
It may be that if direct media is enabled both sides are told to talk direct which works. With Tt this can’t happen so media has to flow through Asterisk, which is not getting the media.
I’d suggest making a complete log accessible with SIP debug (sip set debug on). As well you do appear to have direct media on via “canreinvite=yes” and in the other case the absence of it defaults to on.