That was my point also...
But they sent me lint to rfc3261 12.1.2 and that confused me

So just for resume:

So am I right if I say that in case If provider receives INVITE with Record-Route from my side (myProxy)
Provider should care about it's own Record-Route and it should have uri where myProxy should sent ACK to instead of using Contact in thi case

In case direct conntect UAC -> Provider I still need to use Contact field as URI to send ACK to that explained on the 12.1.2 of rfc3261?