Anyone familiar with SER and its descendants knows the family tree gets a little messy. Basically, here's our understanding of what happened: OpenSER was forked from SER and later renamed to Kamailio. Also OpenSIPS, another popular SIP router, was forked from OpenSER before it was renamed. Though it's no longer in active development, Lintel uses OpenSER's descendants (whether forked or renamed) in most of our clients' telephony products.
We can't claim to be 100% accurate here, but if someone wants to question the SER genealogy, we're more than willing to settle this thing Jerry Springer-style.