asterisk users - Feb 2006 - Help with Grandstream Handytone 386 together with Asterisk and a connected modem

Hello,

we use Handytone 386 adapter together with the Asterisk PBX.

Using normal analog phones together with the Handytone and Asterisk works fine.

We also can connect a standard fax machine to the Handytone ATA adapter.Send and
receive of faxes works fine.

When we connect a standard analog modem to the Handytone adapter we can 
establish outgoing calls but when we try to call the modem we never get an 
answer or see any incoming call on the modem.

What could be the problem?

Thanks and regards,