Did you check the Activity Log right after one of the calls to see why it went to voicemail so quickly. The reason should show in the log.
I'm getting alot of this in the low log setting.
01/19/2018 10:02:22 AM - [CM503003]: Call(C:925): Call to <sip:
[email protected]:5060> has failed; Cause: 487 Request Cancelled/INVITE from 192.168.0.102:5060
01/19/2018 10:02:22 AM - [CM503003]: Call(C:926): Call to <sip:
[email protected]:5060> has failed; Cause: 487 Request Cancelled/INVITE from 192.168.0.124:5060
01/19/2018 10:00:56 AM - [CM503003]: Call(C:920): Call to <sip:
[email protected]:5060> has failed; Cause: 487 Request Cancelled/INVITE from 192.168.0.102:5060
01/19/2018 10:00:56 AM - [CM503003]: Call(C:920): Call to <sip:
[email protected]:5060> has failed; Cause: 487 Request Cancelled/INVITE from 192.168.0.101:5060
01/19/2018 10:00:42 AM - [CM503003]: Call(C:921): Call to <sip:
[email protected]:5060> has failed; Cause: 487 Request Cancelled/INVITE from 192.168.0.124:5060
01/19/2018 10:00:42 AM - [CM503003]: Call(C:919): Call to <sip:
[email protected]:5060> has failed; Cause: 487 Request Cancelled/INVITE from 192.168.0.102:5060
01/19/2018 10:00:15 AM - [CM503003]: Call(C:917): Call to <sip:
[email protected]:5060> has failed; Cause: 487 Request Cancelled/INVITE from 192.168.0.124:5060
01/19/2018 10:00:15 AM - [CM503003]: Call(C:917): Call to <sip:
[email protected]:5060> has failed; Cause: 487 Request Cancelled/INVITE from 192.168.0.101:5060
01/19/2018 9:59:54 AM - [CM503003]: Call(C:915): Call to <sip:
[email protected]:5060> has failed; Cause: 487 Request Cancelled/INVITE from 192.168.0.102:5060
01/19/2018 9:59:54 AM - [CM503003]: Call(C:916): Call to <sip:
[email protected]:5060> has failed; Cause: 487 Request Cancelled/INVITE from 192.168.0.124:5060
01/19/2018 9:59:49 AM - [CM503003]: Call(C:914): Call to <sip:
[email protected]:5060> has failed; Cause: 487 Request Cancelled/INVITE from 192.168.0.102:5060
01/19/2018 9:59:49 AM - [CM503003]: Call(C:913): Call to <sip:
[email protected]:5060> has failed; Cause: 487 Request Cancelled/INVITE from 192.168.0.124:5060
01/19/2018 9:59:33 AM - [CM503003]: Call(C:910): Call to <sip:
[email protected]:5060> has failed; Cause: 487 Request Cancelled/INVITE from 192.168.0.124:5060