I don't think this is supported. I didn't see crystal document listing that but I saw this.
Note Message waiting indicator (MWI) integration is not supported for PSTN access to voice-mail systems at central locations.
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cusrst/admin/sccp_sip_srst/configuration/guide/SCCP_and_SIP_SRST_Admin_Guide/srst_voicemail.html