OK, so what you’re trying to achieve is:
(a) An incoming PSTN call is converted by the 3300V to a VoIP data stream and sent via Ethernet to the Asterisk server where it is handled as an incoming call.
(b) When users dial a PSTN number the Asterisk PBX sends the call over VoIP to the 3300V, where the FXO port converts it to analogue and plays the data down the PSTN line.

This is achieved by:
(a) PSTN lines are connect to FXO ports on Vigor 3300V
(b) FXO ports are configured to an Asterisk/Trixbox server
(c) On the Asterisk/Trixbox PBX server, FXO ports are configured as Trunk Lines

Unfortunately we don’t have one foolproof answer at this time; and we are not knowledgeable about asterisk or trixbox. Three possible answers are:

1) DrayTek Application Note
DrayTek have provided an Application Note about this on their international Draytek web site.
Note that this document shows configuration for both SIP and MGCP – however only one of these protocols can be used at a time. We suggest to use SIP in this case.

2) Extension workaround
In our DDC Australia office, we have used Vigor3300V with FXO port module and Trixbox PBX system.  Our own configuration is described in this file – except that we do not recommend VMware for a production system.
Please note that this involves a work-around.  In order for the Vigor 3300 FXO port to register as a SIP device, it must connect as an extension on the Trixbox system – not as a Trunk line as desired.  Trixbox/Asterisk has a feature which allows calls from an extension to be treated as though it was a trunk line call (I guess this is really for testing) by dialling “7777”.  For incoming PSTN calls this is easily achieved by entering “7777” in the “Hotline Number to Internet:” field on the 3300V’s VoIP > Port Settings page.

Outgoing calls are a different matter.  It is necessary to dial the extension number of the FXO port, listen for PSTN dial tone, then dial the PSTN number.  In our particular case this is acceptable because all our outgoing calls go via our VoIP provider, leaving the PSTN lines for incoming calls.

3) Trixbox Forum suggestion
Other methods may be available in the Trixbox forum.