If your 79xx Cisco SIP phone and the 3CX PBX are both behind the same firewall, then nat_enable needs to equal 0.
nat_enable: "0" (both SIPDefault.cnf and SIPmacaddress.cnf has a nat_enable field, the SIPDefault.cnf applies to all phones)
Also, dont forget:
proxy_register: "1"
in your SIPDefault.cnf file
Some other handy Cisco SIP specific stuff.
Dialplan.xml
This file determines how the off hook dialing will behave.
The dial tone sound for example, timeout after digits, whether or not to connect the call immediately after #, connect immediately after 5xx if your extensions all start with 5 and are 3 digits long, etc.
For example, when the Cisco 7960 goes off hook, you hear an "intercom" dial tone. You can have the dialplan change immediately to the bellcore standard dialtone like you hear in the US on analogs instantly after the 9 key is hit, then finally instantly connect (instead of the timeout), after 1+10 digits, 10 digits, or 7 digits.
It can really confuse people in SIP phones to have to wait for the timeout or remember to hit # after dialing the digits
Here's a good site on the Cisco phones:
http://www.voip-info.org/wiki/view/Asterisk+phone+cisco+79xx
Custom ringtones can be a bit pesky but they are much easier to make/convert in Linux than Windows since there are lots of instructions on the web about making Cisco ringtones using a Linux command line to convert them to RAW PCM files.
One warning on Ringtones for Cisco.... Its not always obvious on other peoples instructions but, the ringtone for Cisco cannot exceed 2.01 seconds in length at 8k samples, 8 bits, mono, and ulaw compression. If its even a fraction longer, the ringtone will be silent on the phone.
DHCP and Cisco...
If you are using a mixture of Cisco SIP phones and non Cisco SIP phones you want not TFTP auto provisioned, be sure to specify DHCP option 150 and use an IP address(es) for the TFTP server. Most other phones look for DHCP option 66 and will ignore option 150.
Or if you want both Cisco and non Cisco tftp provisioned phones getting configs from different TFTP servers, use 66 for non Cisco and 150 for Cisco SIP phone's TFTP server.
I did an install with an Allworx where Allworx phones got option 66 from the server for their auto provisioning, but my client had 25 Cisco SCCP phones to convert to SIP and be auto provisioned from a different TFTP server. I would imagine that 3CX would count that way too.
Cheers!
8)
