[Asterisk-Users] How do I pick up a trailing number in extensions.conf?

[Harald Milz]

Hi,

my SIP provider sends me all the numbers that are dialed, i.e. when a
number is appended to the phone number proper, it gets appended to the
incoming number, like (my number is 0123456789, and I append a "5"): 

To: <sip:491234567895 at sipgate.net>;tag=as1174b008

Question is, how can I use the trailing number in extensions.conf? This is
ideal for a direct dial through to an extension in my ISDN PBX (which I use
as an 8-way ISDN-analog adapter ;-) 

I tried something like

exten => sip,1,Goto(isdn,1${EXTEN:-1},1)

where sip is the extension name in 

register => NNNNNNN:XXXXXX at sipgate.de/sip

But apparently ${EXTEN:-1} is "p" because... how do I do that? 

TIA!

-- 
"On two occasions I have been asked [by members of Parliament!], `Pray,
Mr.  Babbage, if you put into the machine wrong figures, will the right
answers come out?'  I am not able rightly to apprehend the kind of
confusion of ideas that could provoke such a question."
		-- Charles Babbage