Although you hear ring-back tone, the callee side is not ringing.

Please check if the ring tone of the callee is set to off or the volume is too low.

Do you see current session display on the Session List at Brekeke SIP Server?
If yes,

click on [Session ID] and you will be automatically led to [Session Detail] page.
Then check [to-ip] for callee's IP address, [session-status] for current phase (check [phase] if before version 1.5.1.3) and see if it is routed to the correct destination.
If [session-status] indicates "Inviting" or "Provisioning", it will not give a ringing tone to the callee phone.

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System message

Although you hear ring-back tone, the callee side is not ringing.

Although you hear ring-back tone, the callee side is not ringing.