You may have deleted the internal gateway connection. Even if you are not using T1-PRI, you need to configure that, otherwise conferencing and transcoding features will not work. Please see below. One connection of the following type must always be there. Please add it and conferencing should work.
It does not matter whether T1 or E1 is configured. For US, you would configure T1. The default configuration on the PSTN connection should be all that is required.