[Serusers] Asking for advice

Hi, I'm new in IP Telephony world and I began from the beginning using SER ... I have found it very nice, simple to configure/use ... Althoug I've had some problems in redhat9. Ok, some days ago I was talking to a friend of mine who was investigating on VoIP and tall me about VOCAL from Vovida.org and Asterisk ... I have read something about Asterisk in this mailing list but I hadn't know of VOCAL ... Could any one tell me about the open source SIP platforms in the todays world ? Their differences ... Etc ... And also the Oparating Systems usually used for this ... (I work with RedHat Linux 9) I'm working for a really good solution for my University VoIP platform ... Thanks, Robert.