I don’t know why specifying a transport would do that, it should all be the same, but a transport controls what is bound for the SIP signaling and what goes in the SDP. It doesn’t control what is bound for RTP, and by default it will do wildcard so it is then up to the routing table to determine what interface and source IP address is used. If you want to be explicit then an option would be to set “media_address”[1] and “bind_rtp_to_media_address”[2] on the endpoint.