Welcome to the first of many, many problems with getting the 79x0 series phones to work with Asterisk. You won’t believe what happens when you actually get it working with the easily crackable 8 character passwords.
Before you move much further ahead on this, I suggest you check out chan-sccp-b on Github. It speaks to the phones natively, and now that phantomvi has updated the SCCP Manager to work with newer versions of FreePBX, it is almost as easy to use a ECP.