I finally got it working !!
see below the kind of UUI expected by Avaya, the UUI length looks to have a minimum value, I need to dig into it a little bit more into the RFC to understand if I need to calculate a payload to build the UUI
This one works and give me on Avaya phone 1234567890 when I press UUI-INFO button !
SIPAddHeader(User-to-User: 04C80A31323334353637383930FA08269400D55113E3AC;encoding=hex)
Regards