Через provisioning:
<!-- Supplementary Service Subscription -->
<Call_Waiting_Serv_2_ ua="na">No</Call_Waiting_Serv_2_>
<Three_Way_Call_Serv_2_ ua="na">No</Three_Way_Call_Serv_2_>
<Three_Way_Conf_Serv_2_ ua="na">No</Three_Way_Conf_Serv_2_>
Или аналогично вручную.
Для надежности можно call-limit=1 в sip.conf.
