In the Forefront TMG Management console, in the tree, click Firewall Policy.

On the Tasks tab, click Configure VoIP Settings.

Select the Enable internal SIP clients to register externally to enable clients on the internal network to receive incoming calls from the IP PBX.

In the External registration IP address dialog box, enter a network IP address dedicated to the IP PBX system. If you use 0.0.0.0, the Forefront TMG external IP address is used.

In the Number of registration ports for SIP in addition to default port, enter the number of times the same client can register with the external IP PBX.

In the Forefront TMG Management console, in the tree, click Firewall Policy.

On the Tasks tab, click Configure VoIP Settings.

Click Configure SIP Quotas.

In Global max number of registrations on the filter, enter the number of internal clients that are allowed to register with the external IP PBX.

In Max number of registrations for specific IP address, enter the number of internal clients that are allowed to register with the external IP PBX via a specific IP address.

In Global max number of calls on the filter, enter the number of simultaneous calls allowed from internal clients to the external IP PBX.

In Max number of calls for specific IP address, enter the number of simultaneous calls allowed from the internal clients to the external IP PBX via a specific Internal IP address.