Simple SIP proxy

I need to set up a LTM 10.2.4 for simple incoming SIP traffic management:

 

UserAgent ---(external)--> LTM ----(internal)---> SIP Proxy (server)

 

I have configured everything according to the deployment guide (http://www.f5.com/pdf/deployment-guides/load-balancing-sip-dg.pdf, pages 4-8) with only one pool member (SIP server). When a call comes in, the LTM forwards the setup request to the SIP server and the SIP server answers back with a 200 OK to the LTM (its SNAT address). But the LTM never responds anything to the initiating agent. Another problem is that the SIP monitor doesn't work - it downs the server (pool member) although it answers back all right. So I configured a standard UDP monitor instead. Can anyone help me out in resolving this issue? Many thanks in advance, Milan