From a user account that is assigned to the CsUserAdministrator role or the CsAdministrator role, log on to any computer in your internal deployment.

Open a browser window, and then enter the Admin URL to open the Lync Server Control Panel. For details about the different methods you can use to start Lync Server Control Panel, see Open Lync Server Administrative Tools.

In the left navigation bar, click Conferencing and then click Conferencing Policy.

In the list of conferencing policies, select the policy for which you want to enable dial-in conferencing, click Edit, and then click Show details.

To allow users to join meeting by dialing in, check the Enable PSTN dial-in conferencing check box. By default, users can dial in to meetings by using the public switched telephone network (PSTN).

Click Commit.