The extensions.conf config file is mainly used to route incoming connections from remote nodes to the correct node number in app_rpt. It also is used to route outgoing autopatch connections to various channels and VOIP termination providers.
Here is an example of how extensions.conf is used to handle incoming connections for a single node:
[radio-secure] exten => 1234,1,rpt,1234
In the above case, two nodes are defined as asterisk extensions in a “context” called radio-secure defined by a stanza [radio-secure] . An incoming connection directed to extension 1234 will end up calling app_rpt (rpt) with a value of 1234 which should a node number defined in rpt.conf.