It's not just LU, it's also NT. Both have softswitches deployed inside SBC. Sonus also has a token prescence, as does Alcatel. SBC is basically letting the two big boys beat each other over the head while waving the SONS card just in case they don't get the pricing/service point.
If SBC ever goes single-vendor, highest odds are for NT, then LU, with SONS a distant third because it would be cheaper for NT and LU to give the damn things away for free then to let SONS beat them and then have to purchase SONS for the business.