On mobile, sorry for the format.

So here is some context, I work in a sheetmetal union in San Francisco and I'm a 1st year Apprentice. Typically apprentices listen and learn from journeymen, and will occasionally follow the steps of a higher tier apprentice. (Ex: 1st year listens to 4th year at jobsite) This guy we will call Bob is a 3rd year apprentice who never shuts up about how great he is and how the the current foreman and general foreman are preparing him to be a foreman when he journeys out (total malarkey). Still him and I get sent on multiple jobs together where it's just the two of us, and everytime it ends up being me doing all the work while he will sit on his phone watching videos, even when I would ask him for his help his response would be "Nah, you got it, you need practice" and return to his phone. I hated working with him but bit my lip to keep the peace. 

Then came one day where Bob, our general foreman, and I were on a job together for the day. At some point the GF sent me to walk a few blocks away (downtown sf) to another job to grab some materials we needed. It took me about 45 minutes to get there, find the stuff, and bring it back. Immediately on my return bob says "damn that took you forever, did you stop to get coffee or something? You've been gone over an hour." "Uh no I've only been gone 45 minutes and it's because it takes awhile to get there and back" I reply. Later that day as we are walking to the parking garage we come across an old friend of his that is in another trade, I say hello and continue on while bob and this guy talk about hitting a bar. 

When I get home right when I'm about to head to bed I get a call from a blocked number. I thought it was strange and decided to answer it. What I received when I answered was a man that sounded like Bob's friend, who knew me by name, calling me out for taking so long grabbing materials. Saying things like "you suck the foreman's dick. You better hurry up or else. You're a bitch." Just general nastiness and threats. I didnt respond much because I was taken aback and frankly didnt know how to respond. I'm not the type to scream and yell over something so silly and childish, I just wanted to go to sleep. He called me 4 times before he stopped. I called bob and asked him what the hell? He said he didnt know what I was talking about and to stop wasting his time. 

The next day he was an hour late to work from drinking to much and oversleeping. Now I know it was him because only 2 other people were with me that day who would've known how long it took me to grab materials, and the GF wouldnt do that, he's a great guy and we get along great, which led me to believe it was bob and his friend. However, I couldnt prove it. It was a blocked number and I didnt have the foresite to record it. 

I just recently found out he has been let go due to his drinking problems, and to say it put a smile on my face would be an understatement, we haven't worked together in months but I still harbor some ill will towards bob, which leads me to my revenge. AITA for writing his phone number on every portapotty Ive gone into from then until now? With "call for a good time" written next to it? I go into a LOT or portapottys  just due to the amount of jobs i frequent with no other bathroom, is that be an asshole thing to do? Or am I justified in my revenge? 