background: I work in a profession where different departments are in close contact with each other, for the benefit of all parties involved.

colleague B and I hang out in our free time, away from work. we've done so for a couple of years. while in private, B and myself sometimes have a...harsh way of speaking about certain whings (we tell ourselves it helps alleviate stress from our work situation which we both enjoy very much but it pressure is sometimes immense). the things we say to each other are things never mentioned (at least not by me) at work. to me it's just rough humor, nothing racist or hateful. colleague B drives this jargon as much as I do, if not more.

the matter at hand: the other day I spoke with colleague A, whom I have never met and know nothing about, on the phone regarding a certain matter. the conversation was brief and, according to my biased belief, very much professional with me asking for A's input on a certain matter and getting a very good answer to my question. 

now, during this short conversation, I noted to myself that colleague A had a voice that, to me, sounded quite child-like.  I found it somewhat amusing that a competent and grown person had this kind of voice. while I would never say anything about this to A, I did however mention this to colleague B who works in the same department as A. I meant it as a gag of the usual kind and nothing more.

turns out B told A about this, and A, understandably, got upset. I would be too, if I heard shit like that from someone I've just spoken with on the phone. I know B spilled the beans because today I got a physical letter to my desk from A, and the letter goes like this:

&amp;#x200B;

*dear \[OP\],*

*you've acted inappropriately towards me. I'm hereby formally accusing you of being a piece of shit. you will not talk to my colleagues about me in a condescending tone. you sound like a 95 year old fart. I hate you.*

*sincerely,*

*A*

&amp;#x200B;

this letter made me feel like a piece of shit since I never intended that A should know about my infantile joke about A's voice. I also got worried that A might bring this matter to HR (harassment-like behaviour). 

should I apologise to A and, more importantly, am I the asshole?