You would need to ask Tech Support.

Since you do not have access to your SIP credentials, your provisioned ATA is the only device that would be allowed to connect to the main proxy servers (i.e. Central and East). If you enable the Softphone/BYOD access in vPanel, you would then be able to connect other devices to the BYOD server using the SIP credentials provided there. Whether or not you can connect more than 1 device to the BYOD server at the same time is a question for Tech Support. I seem to remember another thread some time ago that if their system detected too many connected devices that it might pick this up as fraudulent activity and won't allow it.