Call me maybe?
nc rev.chal.csaw.io 1001
The binary expects two arguments, the first is an address and the second one a key. If you open the binary in gdb you see that it directly jumps to the address you specify. Okay we need to find an interesting address were we have to jump to.
Open the binary in Ghidra, find the string flag.txt and check where it is used. There is an unused function at 0x400ca0, it has one parameter that is probably the key you provide via second argument and it is compared against a string. You can skip the check and directly jump behind it to address 0x400cbb, so the key does not matter.
./callsite 400cbb 123
flag: flag{you_got_the_call_site}