Hi All,
The following always stumped me:
Quoting The Wiki:
This is not entirely complete.
If no username is available, then Asterisk will try to match on IP AND PORT !
This is relevant to configurations where you receive calls from a provider.
sip.conf (wrong version)
[quote][general]
context=From_SIP ; Default context for incoming calls
…
[Sippy] ; a SER server
type=friend
host=10.1.1.10 ; SER server’s IP address (faked)
context=from-provider[/quote]
extensions.conf:
[quote]
[From_SIP]
; nothing here…
[from-provider]
exten => 5000,1,Milliwatt() ; toooootttt
exten => 5000,2,Hangup()[/quote]
Sip debug output on asterisk:
Found no matching peer or user for ‘10.1.1.10:5061’
snip>
Looking for 5000 in From_SIP <=default(wrong) context
Reliably Transmitting (no NAT):
SIP/2.0 404 Not Found
As you can see, the call lands in the [From_SIP] context, NOT in the [from-provider] context
From this output we learn the Port# the SER is using: 5061
Add this as ‘port=5061’ in the [Sippy] section, reload sip and make a new call:
[quote][Sippy] ; a SER server
type=friend
host=10.1.1.10 ; SER server’s IP address (faked)
port=5061 ; UDP port the SER is listening on
context=from-provider[/quote]
Sip debug output:
snip>
Sending to 10.1.1.10 : 5061 (non-NAT)
Found peer ‘Sippy’ < Ahaaaa !!!
snip>
Looking for 5000 in from-provider < That’s the context I want the call to land in !!
Erm… I’m winking in and out of toying around with Asterisk. The above applies to Asterisk pre-version 2. Don’t know if the behaviour in version 2 has changed in this respect. 
Figuring this out has cost me quite some time, I hope this helps someone
Leo