I hope I understood your question correctly -
Phone is connected to the exchange over ISDN interface to the exchange, exchange is connected to other interface over SS7 interface, say at some point signalling requires to be moved to IP from E1/T1 then exchange is connected to Signalling Gateway over SS7 interface and terminate Sigtran interface on other side. This is the story for the PSTN network.
For mobile network story is same we can assume BSC/RNC as exchange, MSC as exchange, HLR/VLR etc as exchange if they have SS7 interface else can be assumed as switch. And in place of ISDN you have access network to connect UE to these nodes.