3rd Party voicemail

I have a standalone voicemail system (Application is installed on the same machine as Asterisk) . I can forward calls to an extension that would reach this voicemail using SIP but it creates 2 voice channels for 1 call since it is calling out. Is there any way to make Asterisk go to this voicemail as if it was build in Asterisk meaning make Asterisk think that is its voicemail. Or has anyone ever used asterisk with a 3rd party voicemail system?