Honza, obviously you never configured Cisco for SIP, so why did you answer the question? Only to promote your company unneeded products?
Before you give advise, make sure you know what you are talking about.
I have configured many times Cisco with SIP without a SIP Server.
SIP call signaling messages are never initiated by a SIP server, anyone who claims that, should go and read SIP RFC.
If you still have doubts check:
For SIP call flows examples with and without SIP servers.