VoiceMail Server and IVR through Asterisk


I need to have a voicemail/IVR system that can handle atleast 100 simultaneous users who can record,access, listen, delete their voice mails through telephone/mobile via an IVR to this voicemail system. Doing this, what asterisk offers me as default? what further development will I require to get this functionality?
This voicemail(IVR) system should further be able to integrate with an Email server named atMail. Moreover Fax support is also a requirement.

Please reply with any comments/suggessions/queries.