You can actually choose which agent to help if more than one are calling at a time. For PC (with map open) you select the matchmaking tab and then highlight on the map which agent to help. You'll fast travel right to them and auto group.
When we turn this feature off, it should stay off till we decide to turn it back on. It is very annoying to listen to this "call for help" every few minutes.
Hold M to open map with that agent already selected, and then hold W to accept request and wait for loading.
Strangely, WASD is map navigation keys, hence map will move up north!
I'm on PS4 and I found that you open your map and scroll the top menu bar to "matchmaking" with the R1 button. From that menu select the bottom right option "Answer the Call" to join.
I'm not familiar with the Xbox buttons but I assume it's the same process.
Also, from that same menu bar on your map, there is an option second from the right to "Disable On Call" by holding the square button down. I think you may have to disable it every time you log on
Remember you need to disable on call every time you start the game (and that includes when it crashes and you have to restart!) Done it so many times now it's like the first thing i do every session!
On PS4 if the hold down the touch pad it opens the map with the agent selected. I could not read the bottom of the screen so just held down all the buttons until something happened.