A little bit of a controversial opinion but... TL's having a mic/headset is grossly overrated.
Yes, it's beneficial, but most RO2/RS players seem to be under the impression that having a mic automatically makes you a competent TL, and not having one means you're some scrub. In my own opinion most team communication can actually happen amongst any of the players. If I were to have a choice of the one person to have a mic on a team, it'd actually be an SL as they have the best means to communicate what is actually happening. A TL having a mic is nice, but it's not the be all end all of the job. Your job is to throw your smokes, call arty and recon, and find a balance between being at the radio and in the field leading. In my own experience the TL's that talk too much are actually the worst wayyyyyy more often than not. A mic helps ease the line of communication, but it does not do anything to make you call better arty, call consistent recon, throw smokes, etc.
Few things about being TL... when attacking, call arty behind the point to intercept incoming reinforcements. And I mean artillery if possible. Artillery lasts a long time and effectively shuts down an area. Attacking "positions" with arty instead of going for kills is usually the superior option. When defending, don't be scared to call mortars close to your men. I call mortars the "Broken Arrow" option because you can actually call them extremely close to your own guys and still be safe. Mortars are also the fastest and most accurate of the indirect fire options. It also has the quickest cooldown time. This all makes mortars extremely effective at targeting big clusters of enemies. Do not use rockets/naval strike if any of your own guys are within a square vicinity of the siting. They will die. That arty-option will kill everything in the area. It's, of course, extremely awesome, but really should almost never be used. Best use is as a can-opener option at the last point of a game.
Knowing the maps helps a ton. The above mortars option, for example. Let's say you see a bunch of enemies literally right next to your allies. Do you call mortars? Well, if you understand that your guys are inside a house, and that the enemy is stacked outside of it, yes, you call those mortars. Your men will be safe inside the building, but anybody on the other side of it will be eating shrapnel. But you can rarely tell what's actually a "building" or not just by hitting 'M', you have to learn it over time. Knowing the maps also lets you put your own markers down. There are actually a lot of hidden tricks to getting good markers that you really only learn by playing.
Last but not least, have thick skin. TL is very frequently blamed for losses and yet rarely commended for good work. You're like the center on a football team. Nobody knows you're there until you give up a sack. The most frustrating part about being a TL is that you can have a team of awful SL's who never place marks, Rambo-in and die all the time, and just generally leave your team in peril, and you will still get blamed for whatever is happening.
For all the focus on TL's, if you ask me SL1 and usually SL2 are the most important positions in the game by far.