RTP before ACK in SIP?

Hello all,
I am observing SIP exchanges between a Cisco 7940 IPPhone and Asterisk (1.0.9).
And I am very surprised to see that on Invite, Asterisk send RTP packets BEFORE Cisco IP phone send its ACK of the INVITE.
I have the following exchange:

Cisco -----------------Asterisk

INVITE ----------------->
<-------------- 100 Trying
<-------------- 200 OK
<— RTP ---- (Congratulations …)
ACK ------------------->

It seems quite strange, because for me, RTP Packets must be sent after ACK is received, or am I totally wrong ?

It is a problem for me because my firewall between my phones and Asterisk drop the first three RTP packets because they must not be here.

So I hear only ‘gratulations’, instead of “congratulations” …

I used to have a similar problem - but it only happened with certain softphones. I didn’t ever bother investigating it at the packet level though, so it may not have been the same in network terms.

All i can suggest is you upgrade to v1.2 and see if it still happens.