tinc never using UDP for data

[Marwan Gedeon]

Hello list,
I looked around online but didn't find an answer, so if you can help me 
out, I'll appreciate
I'm trying to create a tinc network which transmits data as UDP and not 
TCP, because I will pass VoIP traffic on top of it, which would not work 
fine if using TCP.
I have configured tinc between 2 machines and it's working fine, default 
ports open (UDP 655 and TCP 655) and I have tested with netcat that I 
can transmit data over UDP between the 2 hosts
However, whenever I bring up tinc network,and try to transmit files over 
it, they are going over TCP.
I have read that tinc will try to use UDP for data and automatically 
fall back to TCP, but I can't understand in my case why UDP is not being 
used for data.
I appreciate some help to understand why UDP is not used so I fix the 
problem causing this and make it used for data transmission.
Thank you