Presently, having finished the main game my character doesn't fast travel.
He hitches rides with the BOS flying machines, walks, runs and swims everywhere.
I don't play much anymore as i am waiting for DLC.
However when i do play occasionally my character will be at the opposite side of the map to the call out destination.
He has learnt not to take the Vertibird ride over the entire map because of the consequences.
The game considers flying over a cell as the player being present in that cell, so if the presence of a player in the cell triggers an attack it's unlikely they will notice so it's an instant fail requiring repairs when discovered.
In my game defend the settlement callouts tend to come in pairs.
So if I only get one notification I first return to where I came from and quite often will catch an ongoing attack at my recent point of departure.
From previous experience when i fast travelled all the time, i would quite often find trashed settlements that I had visited recently. Using game saves showed me that seconds after i fast travelled out, an attack had occurred, which didn't qualify for a warning notice because the game considered me to be there at the time of attack initiation.
Presently in relation to callouts I find the game gives you enough time to walk from one side of the map to the other, respond to help defend the checkpoints on the way and deal with any other foes you might come across.
I also diverge a little and visit other settlements and the campsites where those random encounters occur (if it's night time) on the way.
The destination foes are very considerate as they don't actually start their attack until you enter the game cell of your destination. So by walking there it's rare for me to engage with the foes unless I happen upon the second wave of attackers because we are approaching from the same direction.
If you don't turn up then the game punishes you by destroying a large percentage of you improvements.
If you do turn up you receive minor damage which mostly self repairs, ( although I have noticed that market stalls and scavenger stations don't self repair in my game). You can tell they are damaged because they are unmanned and their assigned settlers just stand around twiddling their thumbs.