Internal extensions give SIP error 603 when trying to call

Hi everyone,
I’ve been in the process of installing a new Asterisk system on Debian (with FreePBX on top). After completing installation, I found my internal extensions unable to call each other or the outside. Since internal extension calling is the Hello World of telephony installations, I feel like I’m doing something terribly wrong, but have no idea what.

I’ve included snippets below of my SIP configuration, extension specification, and the output I get when debugging on the Asterisk console. Note the SIP 602 (Refused) errors. Also worth noting is that some PBX functions do work. Dialing *60 on any phone gives the talking clock. So there is some level of connectivity/functionality here, just not much. Any help would be greatly appreciated. Thanks!

SIP Configuration:
SIP Extensions:
SIP Debugging Output: