Line calling in rec tennis can be hilarious.
If you aren't assertive enough, guys get mad and try to roll over you saying "You must not be sure because you didn't sound confident enough"
If you sound overly confident, guys get mad too.
Basically, most guys who want to start issues will just be contrary no matter what you do.
As I've said before, I call the lines as I see them, but give my opponent any point they question. On the two balls that are out that Ian is using for demonstration, I'd them call both out. But if my opponent disagreed, I'd give them both of those points, even knowing full well the ball is out. Just makes life easier. I really hate to deal with "Are you sure?" guys. But, if they ask, I always tell them I'm sure of the out call I made, but I'll go with your call and call the ball in. Then I always say the score (to signal that the discussion is over and we are moving on to the next point).
99% of people I play just play my call without making any issue. Most people generally understand that you are trying your best, but you might make mistakes (or we might just see things differently). The 1% of people that "get off" on questioning line calls, I just give them the point. I know that engaging with them in any way is more disruptive (and ultimately worse) than just giving them a point they didn't earn.
Some people really do come to the court looking to argue about something in every match. Not many, but there are people like that out there. The few I've run into, I just let them have their way in everything and apologize for all of the things that I'm supposedly doing. I've never had a person continue with this stuff when it's clear I'm not going to engage them and argue with them in any way.