The two devices shouldn't both need to be on the one port 5061: we have numerous solutions to that problem at this point, from specifying the port as part of th

The two devices shouldn't both need to be on the one port 5061: we have numerous solutions to that problem at this point, from specifying the port as part of the connection to auto-detecting it using SRV in DNS. Yes: making some small changes to existing software to not assume hardcoded ports is required in this future... but there is no way that those changes would have been (or frankly even still are) harder than the insane solution of reimplementing and redesigning the entire world--requiring its own changes to every protocol (including SIP) and breaking a strict and extremely large superset of the same references that merely needed a port number--to support IPv6.