"HOLD" -- Yeah, I know, everyone should know it, but it never hurts to remind everyone that the call of a hold is a very important safety measure. When someone is possibly injured, someone calls a hold and everyone within earshot on the field stops fighting and takes a knee, and then repeats "HOLD".
"MIND THE GAP" -- You'll hear this call usually from someone in the middle or next to a spacing gap. They are trying to get reinforcements to spread out to cover the distance.
"LEFT SIDE IS FOLDING" -- The left side of your line is getting crushed and rolled around. See this post all about the subject.
"HURRY UP RIGHT SIDE" -- The left side is in trouble and is forced into fighting defensive, they need the opposite end of the line to push (see below) and make something happen.
"PUSH" -- Contrary to some views, this doesn't mean rush in with reckless abandon. Pushing is stepping up into range and swinging, with a slow, determined pace. You need to force action by moving the whole section into the fight, rather than maintaining the stalemate.
"PUSH THROUGH" -- Rather than just pushing up to the line, they see an opportunity to force some fighters through a gap. A couple of people may not make it, but you may be able to cause a minor/major breakthrough.
"CHARGE!!!" -- Still, not quite recklessly, but force your way through their lines and try to take a bunch of them with you. If you survive, you'll be in their backfield or have all of them dead. See this post for a little more info.
"ROLL RIGHT" -- This is a flanking maneuver. They want you to circle around the flank and try to fold it back, ie. rolling the flank.
"SLIDE/SHIFT RIGHT" -- Often used to fix spacing gaps, they just want you to shift down the line without changing the distance to the enemy.
"GAP LEFT" -- This is more likely letting you know there is a gap in the enemy line that can be exploited. If the person yelling this is engaged with a target, they might be suggesting you go through while they keep the gap open.
"SPEAR RIGHT" -- There is a spear moving in from your right, prepare to block about 5 seconds ago.
"STAB/LOOK LEFT" -- This is a common phrase I use to let my support know there are open targets on my side that they may not be able to see around me. This is usually a vision gap or staggered gap they could exploit in the enemy line.
"WATCH RIGHT" -- An enemy has stepped into a good position a step or two out of range to your right. They could possibly kill you in a matter of a few seconds...or less.
"BEHIND" -- Enemies are in your backfield, might want to look behind you or even block your back.
"[your name here], BEHIND" -- Someone doesn't like you and is about to backstab you, definitely back block and hope for the best.
"ARROW UP" -- An archer has their bow up and at least partially drawn an arrow.
"ARROW/JAVELIN" -- Projectile is airborne. Look to block or cover. Stupid archers...
"I CALL YOU OUT" -- They want an honor duel, one on one without interference. Squires are generally obliged to accept when called out by a knight.
"RED, DOUBLE,REDREDRED" -- I have a glaive and I'm still alive! Read this post to see what you can do to help.