Identify the state owner
A registration front end can reduce or shape the registration traffic sent to downstream systems, depending on the selected mode and implementation. OpenSIPS includes a mid-registrar module for this role. It changes the relationship between endpoint refreshes and upstream registration state, so expiry and contact behavior need explicit testing. Document whether the edge, downstream PBX or another service is authoritative for each binding. Do not assume that observing a REGISTER on one side means an identical request must appear on the other.
One possible service architecture. Asterisk or FreeSWITCH supplies call applications; the proxy/registrar handles signaling policy. Media anchoring and state recovery must be designed explicitly.
Route calls according to responsibility
OpenSIPS offers routing and load-distribution modules with different purposes. Choose according to the resources and state the application tier needs, not merely the desire to alternate requests between IP addresses. A new call can be assigned to an available application server while later requests must still reach the correct dialog state. Authentication, destination authorization, routing selection and media relay remain separate policy decisions.
Test refresh and failure together
Register a pilot endpoint, make an incoming call, let refresh cycles occur and then take a downstream node out of service in an authorized test. Observe which state survives and how new requests find a usable destination. Repeat after endpoint network change and credential revocation. Record the module version, selected operating mode and observed expiry behavior. Keep configuration examples from other branches as reference material until their semantics have been checked against the installed 3.6 module documentation.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.