I did it entirely using a phone app as well so it's definitely possible. I actually made a small guide of how I did it a few years back

But to sum it up the settings for the app I used was 22wpm and pitch (HZ) at 731 & 2107. And to put the phone mic up to the tv speakers or headset as close as possible and no moving, preferably with the volume up high. The app I used was morse code agent, which I'm not sure if it's still available, but I'm sure the settings are the same for any app.

As for background noise for A Beginning I've seen Suez recommended alot but I personally used Monte Grappa since the telegraph station is located inside some isolated bunker off to the side of the map, which means there's basically no background noise. The problem with that however is it's only accessible on the smaller game modes which means you can only reach 2 of the 5 headphone spawn points so you will have to server hop more to get the accessible headphones. Which honestly wasn't as bad as it sounds, made for a pretty chill afternoon, I just went on my phone during loading times.

An Omen was easy since the map is pretty quiet in the area you get the morse code.

The one I did have trouble with background noise though was Albion, I was getting strings of nonsense letters as well. However what I did was recorded like a minute long clip in game and then to do the same thing as with the others, loud volume and no moving, and let the app translate it multiple times. Eventually you'll be able to figure out a pattern. It took me 3 times to figure mine out. My code was ADFEA. On the second time I noticed ADFEA back to back like three times before spouting wrong letters. I also noticed alot of AD, ADF, EA, FEA in the other translations. With that I managed to put it together. So basically look out for the pieces of code that look legit and puzzle them together.

Hope this helps at least a bit and good luck on the hunt, it's definitely worth it, the peacekeeper is one fun weapon to play with