magnus,
From what I can tell they are not following RFC 3261, but I also don't see where you are requiring PRACK (Support 100rel) either. PRACK support is only used for acknowledgement of provisional messages (100, 180, 183, etc..) where a response is required 200/ACK.
The 180 Ringing is a message from the UA (Babytel in this case) showing that they received the INVITE and are trying to alert the user. This response MAY be used to initiate local ringback.
In contrast a 183 Session Progress response is used to convey information about the progress of the call that is not otherwise classified (read we don't know that the remote phone is ringing); this usually comes with SDP as well so that messages can be played while waiting.
Now, it looks as though we are not seeing all of the traffic. Where did you get the packet capture from and what were the settings on that device for recording the packet information? You could also request a packet capture from Babytel and see if they are sending out a Provisional Message of 180 Ringing.
Thanks,
TE