MWI Notify Error on Audiocodes MP-124

Hi,

I?m having problems configuring MWI on my Audiocodes MP-124 FXS. When my Asterisks send the “Notify” command to the Audiocodes, It responds with a "481 Call/Transaction Does Not Exist"
I?m using Asterisk 1.2.9.1.

Do you know what might be the problem. I?m also using a Linksys PAP2 and is working fine.

Regards,

Mariano