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.
Configure Multicast Paging on the Cisco IP Phone 7800 Series or 8800 Series Multiplatform Phone
The Cisco IP Phone 7800 and 8800 Series Multiplatform Phones provide voice communication over an Internet Protocol (IP) network...
Add Call Park on a Cisco 7800 or 8800 Series Multiplatform Phone Key Expansion Module
Call park allows the user of the phone to put an incoming call on hold so that the call can be retrieved on another phone. A call is park...