My attempt at a storyline.
October 21st, 2077
(Air Raid Sirens go off. The Great War begins and lasts two hours)
Calling any station. Calling any station. Calling any station. This is OZ (Bridgeport). Please stand by.
Bridgeport, this is Concord (MA), message: Quabbin Reservoir is hit, repeat, Quabbin Reservoir is hit, over.
(Boston is hit by a nuclear warhead. The NCRA was in the process of being updated with a situation report)
"National Catastrophe Relief Auxiliary Unit, Boston 314, 7 Emergency Message: NEW YORK STAGE 7 WASHINGTON STAGE 7 LOS ANGELES STAGE 7 AUSTIN STAGE 7 CHICAGO STAGE 7 NEW ORLEAN STAGE 7 LAS VEGAS UNKNOWN MIAMI STAGE 7 DALLAS STAGE 7 DENVER STAGE 7 SAN DIEGO STAGE 7 SAN FRANCISCO STAGE 7 BOSTO-"
(A vault technician opens 119 to any survivors of the initial attack. IT is possible 7667 is the code needed to enter the Vault)
Calling all stations, calling all stations, calling all stations, this is Vault Technician 2510730 one six emergency Vault-Tec shelter for survivors in Quabbin, number 119, repeat, emergency Vault-Tec shelter for survivors in Quabbin, number 119, over
(The radio operator who attempted to reach the NCRA realizes Boston is now gone and that there is no hope)
Calling all stations, calling all stations, calling all stations. Boston is gone, I repeat, Boston is gone. No Hope. Frequency 3450 Return Transmission. Goodbye America.
(Six days pass, and the Nuclear Winter occurs)
TIME ELAPSES
July 4th, 2299
(The Survivor heads to the Institute for help)
MY DEAR SISTER. IM HEADING TO THE INSTITUTE. GOD KNOWS WHAT HAPPENED TO THIS PLACE NOW. THIS OLD BASTARD WILL HELP US. HE MUST HELP US. 0321
(The Institute refuses to help the survivor. He decides to venture to Black Row.)
THE INSTITUTE IS SEALED. THEY WON'T HELP US. I'M HEADING TO BLACK ROW. -NGQHT 5120-
(In the meantime, Vault 119 is being attacked)
"Calling all stations, this is overseer 119 emergency enemy unknown, SOS. Vault 119 Quabbin 7667, repeat, SOS, Vault 119 Quabbin 7667 enemy unknown."
(The attackers manage to breach the vault doors. The vault dwellers retreat to level 4.)
"Calling all stations, this is Vault Overseer 119, anybody near Quabbin? SOS Vault door breach, repeat, SOS Vault door breach, enemy unknown, level 1, 2, and 3 gone, survivors on (service lift to OR in secure location) level 4"
(The attackers manage to kill the overseer (whose code is 15334), who probably attempted to defend his vault dwellers and hold off the attackers. The attackers now attempt to breach Level 4)
"Calling all stations, this is Vault 119, SOS, Level 4 is under attack, Overseer is dead, repeat, SOS, Level 4 is under attack, Overseer is dead. We wont make it."
(The Survivor has reached Black Row. They send our a Quick Response Force to help at Vault 119.)
"Calling Black Row Out Station. Ground is safe. Too Late. Level 4 breach. No survivors, 899 corpses. We are leaving Vault 119. Over, back to you."
EDIT 1: I like the idea of OTST meaning Out Station. And while I don't know what TL means (since "To Land" and "Total Loss" also make sense), I am sticking to "Too Late".
EDIT 2: I am pretty torn. On one hand, the stasis idea is supported by the fact that the Tranquility Lane music played on the original number. However, I also am starting to think that the Survivor was not around during the Pre-War messages. It is possible that the Survivor is the descendant of someone who was saved by the Vault Technician - perhaps the offspring of the Vault Technician themselves! Maybe that is the irony. Your great grandfather or whatever opened the Vault to save people (900 to be exact) and now, 222 years later, you are tasked with saving the Vault Dwellers under attack. However, you fail, and your sister is killed. You are now attempting to track down those responsible for the attack.
EDIT 3: Not that I think I need to clarify this, but you are the 900th person.