Communication between servers within the core side of the network is typically trusted and usually clear-text/un-encrypted, even if it was originally encrypted at the edge.
On the interconnect side of the SBCs, the remote peers may or may not be trusted. Some interfaces maybe in clear text/un-encrypted, others may use IPsec or TLS to protect the interface. Scaling is much less of a concern than at the access side as there are typically far fewer peers.