Generally, top telcos use software made by top telco's software vendors... of course
Say in other words, top telco's equipment vendors have their own sw team or third-party suppliers. Equipment vendors are Alcatel, Lucent (ex-AT&T), Ericsson, Nokia, etc. You can see at those companies' web site to check.
Which kind of software:
- system and service management;
- switching, IMS, etc;
and so on.
Hope it works.