[Serusers] SIP / H323 connectivity

Hello I want to have a server that supports SIP and H.323 endpoints calling eachother, so I installed GNUGK, SER and Asterisk. GnuGK is needed for H.323 endpoints and users Accounting, Authentication and Authorization. SER is needed for the same 3 A's but for SIP users. Asterisk has some good funcionalities like voicemail, call holding and PSTN connection, supports SIP and H.323, but its not a realy gatekeeper and a realy SIP server. Now, Who is going to accept the SIP calls, Asterisk ou SER? Who is going to redirect calls to PSTN? Asterisk or SER? In the H.323->SIP calls, who should the gnugk contact? Asterisk or SER? If someone is using a similar network, please give me a little help. Thanks Joao Pereira