I was trying to change the SIP account of our previous administrator, because he never gave us his password, and I want to set myself up as the default user and have full access to the eyeBeam software, but I keep getting a registration error 403.
The error you encounter means you have no access rights to change the settings of the SIP account. You still have to contact the Administrator to give you permission to do so. If you require official support from developers, then I recommend opening a ticket on their support platform. Follow the link below and state your problem then someone from their team will help you reset the password to assign yourself as a new admin.
Support platform: https://support.counterpath.com/ticket/add