What goes wrong? Did you remember to have the device send to port 5160 (or whatever you have bind port set to)?
Other options:
Set up your device as a pjsip trunk w/no registration or authentication. You’ll be able to make and receive calls, but you won’t have voicemail, follow-me, forwarding, DND, etc. What kind of device is this? A trunk might be more suitable.
Set it up as a custom extension and put the config in the pjsip custom config files.
Possibly, you can modify the existing config as shown in