What @billsimon said, but remember that Cisco 79xx phones are among the most challenging SIP phones available when it comes to provisioning. EPM isn’t up to the task, and the SIP image for these phones is limited by the lack of hardware resources in the phone.
Having said that, you need to look at the logs in /var/log/asterisk/full to see what the actual problem is. I’m going to guess that your password is too long (I think 12 characters is too many for many SIP loads). This security risk is another reason that these phones are not recommended for use as a SIP phone.
It is, by a huge factor, the problem with someone says “I’m using EPM to configure a Cisco phone.”
The phones hate SIP anyway, and EPM assumes that the SIP device it’s managing is at least “friendly”.
@amrsalime, the point about the password is also a warning. If you allow SIP access to the outside world, you can expect this SIP extension to be hacked within about 72 hours. This is one of the reasons I recommend against using the SIP load and wrote the instructions for installing the Chan-SCCP-B Channel Driver for Skinny phones. It gives you a fully functional phone with state-aware horizontal buttons and other cool features that work without actually hacking Asterisk source.
Search “FreePBX install chan-sccp-b @GitHub” for more information.
This is not in any way related to EPM. The SIP secrets are generated independently of EPM and you have the same issue if you are manually configuring the Cisco endpoint. @amrsalime note that the generated SIP secret length is settable in Advanced Settings.
If the question also includes words like “Cisco”, “new install”, “can’t register”, “using EPM”, and “Newbie”, my assumption is that they are probably getting jammed up on the “default” 32 (IIRC) password.
You are absolutely correct that this isn’t the “fault” of EPM. It’s just a feature.