Those third-party vendors can be a problem.
Outsourcing is very common in today's business world. It is not uncommon for a complex weave of systems to be networked together. The problem with that is when there is a problem, you likely don't have immediate visibility into where the problem is originating.
This is likely the case with CenturyLink, who is pointing the finger at one of their vendors, see CenturyLink’s recent outage is being investigated by at least two states, in addition to the FCC.
Here in Washington state, CenturyLink was already fined several million dollars for a previous statewide 911 outage. I expect more fines will be coming.
Steve Myers shared the link above.