DO tell people where you are (as medic) so they can come protect you
DO indicate your intended path of travel
DON'T tilt or go silent

edit: also there are resources in this thread.

remember that the flank usually can't see what the combo sees so you need to give them a lot of information about what's happening, so they can make appropriate plays

also if you are a medic trying to main call or just have better comms, call out everything that your pocket or heal target is shooting and it will do wonders

Call your location on the map so people will know where are you
Call when you're about to spawn/when you spawned so people can meet you
Call what you want your team to do - do it loud
Call your uber percentage. You can also call how much seconds left to build an uber, thats a personal preference
#2 linked a very good thread, you might want to read it

Call to push when you have uber advantage
Call to push when you have number advantage

As you get more experienced, you will see more opportunities to effectively use uber/number advantage. You can still push if you have uber disadvantage because of a number advantage. You take ground with the number advantage in order to get a force. If you have uber advantage, you can sometimes catch out a team even if you don't have number advantage. For example, its 3v6, but you play close to a choke and uber onto the other team when they overextend. This can work in your favor, but your team just has to hit their shots.

assert your authority

BE LOUD

YOU DA BOSS

This VOD is from Paulsen's stream last night.

http://www.twitch.tv/paulsen__/v/33521288

I'm not sure if the scrims start right away but listening to their main caller Brutalix (Medic) should give you an incredible amount of information on what you can be vocal about and how to control your team etc...

When to call to push

Contents of a Main Call:

• Direction the combo is moving
  
• Who is leading the push
  
• What the goal of the push is (capture ground, kill something, force the medic, etc.)

Advantages (even major ones) in TF2 are fleeting, try to make a maincall as soon as possible.

Don't worry so much about making the optimal main call. Provided all 6 team members are on the same page and execute the main call you will be successful against all but the best teams in the world.

Discuss your maincalls with your team after each game. The only way you will improve is with practice and constructive criticism. Do not discuss them during a game.

Do not push without a maincall, no matter how trivial it is. No main call means everybody on your team is assuming what to do and unless your team is you and 5 of your clones there is always going to be some misunderstanding of how to play a situation.

Give your team five seconds after making a maincall to be in position to push. This pause gives everybody time to stop what they are doing, get into position, and voice any concerns they may have. The number one cause of failed pushes is trying to move too quickly after a maincall.

Be authoritative with your maincall. Teammates that ignore your maincalls are prime candidates for being cut. Discuss with your team members that no matter how stupid it sounds, if you make a maincall they must follow through.

Finally, if kevin ever decides to upload the master of classes, pyyyour had an hour long talk about communication that you should watch.

https://www.reddit.com/r/truetf2/comments/3tf2rc/catalogue_of_master_of_classes_guides/

http://www.twitch.tv/teamfortresstv/v/25773006?t=180m39s

Make sure when youre calling that you are decisive with your calls. Being skeptical or having doubt just makes things a mess. Constantly going back on a call you just made pretty much kills your coordination.

Hype your team up

what does "tilt" mean?

cozens post p much gets it

just make your call confidently and don't waffle

a shitty call can work if everyone is doing the same thing and the perfect call can fail if half your team isn't in on it

obviously you wanna try to make the right call all the time and you'll be considering what to do nonstop, but when words come out of your mouth, make sure they're confident and clear.

To choke, to fail a play

more like

Gambling term descibing an angry or "reeling" state of mind usually caused by a big or unluck loss. People playing on tilt play below their usual abillity and usually make larger and more aggressive bets to try to quickly recover their loss.

When you're a med, call stuff that it's important for the flank to know in terms of whats going on in the combo.
One of the biggest things you need to be calling is when you've ubered in, it's really helpful for the med to call whether the uber's good or bad and if you need people with you in/after the uber. If your uber is shit, make sure you make it clear for your flank so they can get the fuck out.