In Manager, System > Telephony > Telephony.
Set the “Maximum SIP Sessions” value greater than 0.
Manager Help provides the following information for the setting;
Default = 0.
This field is shown for Server Edition systems. On Server Edition systems, the Maximum SIP Sessions value must match the total number of SIP set and trunk calls that can occur at the same time.
The Maximum SIP Sessions setting determines the number of SIP Trunk Channel licenses reserved for concurrent sessions on any SIP trunks provided by the server. Those licenses are reserved from the pool of SIP Trunk Channel licenses in the configuration of the Primary Server.
Case 2: This setting has change location and name in 10.0 and higher releases. It is now SIP Trunk Sessions, located under License > Remote Server Tab within IP office Manager;