Asterisk with external/3rd Party Voicemail

I am at a loss and could not see this answered. Sorry if this is a redundant question that is answered somewhere. I have been looking for the last 3 hours searching the forums and web for an answer to the question below.

We have a 3rd Party Voicemail system with an older phone system. We want to deploy Asterisk with our AVST CallXpress voicemail. Is there a way to make Asterisk intergrate with an external voicemail system?

Any information even general would be appreciated.