In a nutshell, the problem appears to be occurring in rural areas.
Where long distance or wireless carriers normally pay higher-than-average charges to the local telephone company to complete calls.
That is, in order for a long distance or wireless carrier to complete one of its subscriber's calls to a resident of a rural area, the carrier
must get the call to the exchange serving that resident (the local phone company), and then pay a charge to that local carrier to access
its exchange. The physical process of getting the call to the exchange is called "routing," and the charge paid by the long distance
company to the local carrier is called an "access charge." These charges are part of the decades-old system of "access charges"
that help pay for the cost of rural networks. To minimize these charges, some long-distance and wireless carriers contract
with third-party "least-cost routing" service providers to connect calls to their destination at the lowest cost possible.
Although many of these contracts include strictly-defined performance parameters, it appears that all too frequently those performance
levels are not being met or, indeed, some calls are not even connecting at all.

The FCC is addressing call completion and call quality problems affecting long distance, wireless, and VoIP calls to rural telephone customers on multiple fronts.

In October 2013, the FCC adopted new rules that will strengthen its ability to ensure a reasonable and nondiscriminatory level of service to rural areas. 
These rules are designed to improve the Commission’s ability to monitor providers’ delivery of long-distance calls to rural areas and to aid
the prosecution of violations of the Communications Act.  We also adopted a rule that will prevent the caller from hearing ringing before the called party's
phone in reality is ringing.

This is  still an ongoing problem and  may take  months  to  correct.