The sources to allow such a get are the mishna on Gittin 66a (translation and interpolation by R Steinsaltz)
With regard to one who was thrown into a pit and thought that he would
die there, and he said that anyone who hears his voice should write a
bill of divorce for his wife, and he specified his name, her name, and
all relevant details, those who hear him should write this bill of
divorce and give it to his wife, even though they do not see the man
and do not know him.
The gemara a bit below comments
A Sage from the school of Rabbi Yishmael taught: During a time of
danger, when there is the likelihood that the wife would assume
deserted wife status, one writes and gives a bill of divorce even
though the people instructed to do so are not familiar with the man
who gave the instructions. Here too, when a voice is heard from a pit,
one writes and gives the bill of divorce, as there is no possibility
of properly clarifying the issue.
The Rambam codifies this as halacha in Hilchot Gerushin 2:13.
So in case of emergency there is no need to appear in person and someone who knows the caller (in times of danger: even someone who doesn't) would be able to start the get process.