How mod_verto is invoked to handle incoming call?

[kaiduan xie]

Hi all,
I am learning Freeswitch webrtc, and setup an environment with demo configuration as below.
1. User 1008 is logged in from browser with Verto.
2. User 1000 is logged in from X-Lite SIP phone.
"sofia status profile internal reg" only shows 1000.
1000 makes a call to 1008, 1008 answers the call, and call is established between browser and X-Lite.
The following is from freeswitch log, 
2662 7fbee0e2-b766-11e8-b3d6-f9bb4821b253 2018-09-13 15:05:49.296720 [NOTICE] switch_ivr_originate.c:2868 Cannot create outgoing channel of type [error] cause: [USER_NOT_REGISTERED]

2663 7fc69cd8-b766-11e8-b3f8-f9bb4821b253 2018-09-13 15:05:49.296720 [DEBUG] mod_rtc.c:392 () State Change CS_NEW -> CS_INIT
I am wondering how mod_verto is invoked to handle the incoming call. Can someone explain please?

Many thanks,
/Kaiduan
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