[asterisk-users] sip channel error - extension pattern matching problem

Hi,

When I have the following extension matching defined:
exten => _an_.,1,NoOp(-- Context routing-sip-announcement for ${EXTEN} --)

Asterisk doesn't find it when it receives such SIP request:
<--- SIP read from 192.168.129.38:7160 --->
INVITE sip:an_hellboy@ms.sip.rd.touk.pl SIP/2.0
Record-Route: <sip: 192.168.129.38:7160;lr=on>
...

for instance when I use
such extension:
exten => _vm_.,1,NoOp(-- Context routing-sip-voicemail for ${EXTEN} --)

Asterisk finds extensions for RURI like:
<--- SIP read from 192.168.129.38:7160 --->
INVITE sip:vm_hellboy@ms.sip.rd.touk.pl SIP/2.0
...

Is this an error?
What did I miss off not?

Thanks in advance
Tomasz