"In the telcos, singlemode fiber is used to connect long distance switches, central offices and SLCs (subscriber loop carriers, small switches in pedestals in subdivisions or office parks or in the basement of a larger building). Practically every telco's network is now fiber optics except the connection to the home." [1]
Specifically, AT&T has transitioned its PSTN backbone fully to SIP, which is a form of VoIP: "AT&T will exchange SIP traffic at the access border controller layer (i.e. SBCs, more specifically Acme Packet SBCs) via IP handoff at a few "strategic locations," directly peering with a select number of Tier 1 carriers. AT&T will also provide transit and direct termination through its network and support all roaming traffic to interwork with other wireless carriers" [2]
So both voice and data are going over the same fiber lines, since it's all IP (which reduces costs considerably.) We know that Room 641A was one of many QUANTUM locations, where traffic was beam-split. [3]
It seems from [4], that we have confirmation that TURMOIL and INCENSER snarf up PSTN as well as IP. Since all of that was working well before 2010, and we know TEMPORA acts as a buffer for high-volume stuff (fed by the Narus machines), and voice is IP now because of SIP, I feel reasonably confident it's getting stored at some duration at the exchanges.
1. https://www.lanshack.com/fiber-optic-tutorial-network.aspx
2. http://sip-trunking.tmcnet.com/topics/enterprise-voip/articl...
3. https://en.m.wikipedia.org/wiki/Room_641A
4. https://electrospaces.blogspot.com/2014/11/incenser-or-how-n...