asterisk users - May 2006 - Selecting the outbound port from FXO device

Hi,

I am using Wellgate 3806 FXO(SIP Proxy Mode) with Asterisk. I have 6 
lines registred as

SIP/2901 (line1)
.
.
SIP/2906 (line6)

to the asterisk. I have a dialplan configuration like

exten => _90NXXXXXXXXX,1,Dial(SIP/2902/${EXTEN:1})
or
exten => _90NXXXXXXXXX,1,Dial(SIP/${EXTEN:1}@2902)

When I call a number matching the above pattern, the first line (#1) of 
my FXO device is activated although I had chosen the second line (#2).

On the other way, with a configuration like below

exten => 72,1,Dial(SIP/2902)

I call 72, and the correct outbound line is activated and I get the dial 
tone from line.

I want to choose the outbound port of the FXO device while one stage 
dialing. How can I do that?

Your help is very appreciated.

Thanks,
Saki