Asterisk to Altigen connection


I would like to connect a remote telephone to my Altigen system using the IAX2 protocol. Altigen’s VOIP implimentation is problematic and fragile so I would like to replace it with IAX2 untill I can replace the Altigen system entirely. I have set up an Asterisk box with a few of the $15 FXO clone cards. The combinations seems to work well enough but control can be a bit confusing and voicemail gets left on the Asterisk box. What I really need, for now, is a transparent connection between the Altigen extension port and the remote telephone. Is there a way to configure Asterisk to behave this way ? Is there another solution that may work better for this temporary installation ?