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Confirm if the phone is under protection of firewall or something like that.
Confirm if the phone’s Check SIP User ID function is activated (Profile → Advanced → Check SIP User ID → Always Check).
You may need to enable this setting for all profiles.
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Further Support
If the registration issue still exists, you need to create a ticket and provide us the debug filesproblem remains, please contact Htek Support (support@htek.com).