From AOPA Online:

When the FAA ended the practice of relaying IFR clearances and cancellations between pilots and air traffic control through Flight Service at locations where direct radio contact between an aircraft and ATC is not possible, numerous pilots asked AOPA for more information about using the new direct phone numbers to ATC pilots must now call instead.

AOPA Online 7/10/2019, by Dan Namowitz

The main change to note is that Flight Service outside of Alaska no longer relays clearances to pilots from ATC, nor does Flight Service relay IFR fight plan cancellations to ATC in places or at times when it is not possible to call ATC on the radio before takeoff or after landing. Instead, phone numbers are being added to the airport listings in the chart supplement (formerly the airport/facilities directory) for pilots to use to call ATC to receive clearances and cancel IFR flight plans. Look for a line in the airport listing headed “Clearance delivery phone” at airports where the service is available

AOPA Online 7/10/2019, by Dan Namowitz