I guess just experience? I used to struggle with Indian and African accents but I’ve worked with enough people who speak with them and now I understand them most of the time. Don’t be afraid to ask him to repeat himself (as long as it’s not excessive). You could use the mic quality as an excuse if you feel uncomfortable commenting on his accent. I try to make it a point to say I respect their ability to learn english since I only speak one language and they speak multiple (which is badass) but that sometimes I have a hard time understanding certain words because I’m not used to hearing them sound that particular way. The man knows he has an accent unlike what most of his colleagues have, I don’t think there’s anything wrong with acknowledging it.