If they don't answer the phone, call the local FSDO and tell them that:Easy to do, if they answer the phone!
I tried calling XYZ tower to provide them with notice of a small UAS flight x-miles from the airport pursuant to Section 336 of the FAA Modernization and Reform Act of 2012, but since they don't answer the phone, I am notifying you.
When they say that they "can't approve the flight", remind them that the law in Section 336 of the FMRA only requires that ATC be notified prior to the flight. You have been notified.
When they say "We aren't part of ATC", then ask for the phone number of the nearest ARTCC (Air Route Traffic Control Center).