Lync 2013 call forwarding (working and non-working scenario)

Question

Hi guys!
We are trying to solve a non-working scenario in Lync 2013 voip calling.
SCENARIO 1 that does not work:
Person A with a mobile phone calls Person B who's telephone number is not located on Lync SIP TRUNK. Person B answers the call and then he wishes to transfer this call to Person C. He call Person C until he hears call ringing tone from him
and then he transfer this call to him. When Person C answer the call he does not hear Person A and call get's disconnected after cca. 20seconds.
SCENARIO 2 that does work:
Person A with a mobile phone calls Person B who's telephone number is not located on Lync SIP TRUNK. Person B wishes to transfer this call to Person C who's number is located on Lync SIP trunk. In this scenario he first wait that Person C
answer/accept his call, then he transfer call from Person A to him. This way Person C is able to hear Person A.
Anyway how to figure it out if this is a problem on provider side or on Lync 2013 server side?
with best regards,
bostjanc

Answer 1

Refer support wasn't set to none.
If we change it to none, and restart Mediation service, unfortunatelly there are no differences.
We have also tried editing: C:\Program Files\Microsoft Lync Server 2013\Mediation Server\MediationServerSvc.exe.config
FROM:
<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <runtime>
    <generatePublisherEvidence enabled="false"/>
  </runtime>
</configuration>
TO:
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<appSettings>
                <add key="20.20.20.20.ReferredBySupported" value="true"/>
    </appSettings>
  <runtime>
    <generatePublisherEvidence enabled="false"/>
  </runtime>
</configuration>
But that didn't do anything useful.
bostjanc