You need a local linux expert
You could try running lsof on the process to try and find the pathname for the executable (this is likely to be work best immediately after booting).
What does “sudo find / -name asterisk -print” return?
Also, something in the startup scripts must be starting it, so look where they are kept for your version of Linux.
However, if you have big pieces of optional software running that you didn’t configure, you should be seriously worried about the integrity of that machine and you really do need a linux expert to work out if it has been compromised (although I can’t think why a hacker would install Asterisk).