[CC] Echoed Voice 60
During your next turn, this Pokémon's Echoed Voice attack does 60 more damage (before applying Weakness and Resistance).
"does 60 more damage" adds damage to the base damage, 60 here. It basically checks if you used the attack last turn, if yes, bonus damage applies. There's no increment.
None of you is right here. The damage goes 60-120-120-120... if the attack is used every turn.
This follows official ruling from The Rules Compendium EX:
== EXTRA COMET PUNCH (Metagross ex - EX:Hidden Legends)
Q. Metagross-EX's "Extra Comet Punch" attack says, "During your next turn, E.C.P. does 50 damage plus 50 more damage". Does this mean that on the first turn I do 50, then on the second consecutive turn 100, then on the third consecutive turn 150, then 200, etc.?
A. No, but it does mean that whenever you do Extra Comet Punch in consecutive turns you add +50 to the base damage of the attack. So on the first turn you would do 50, then second turn does 100 (Base 50 plus extra 50 from previous turn), then third turn does 100 (again, Base 50 plus extra 50 from previous turn), etc. But don't forget, benching or switching Metagross-EX ends this effect! (Jun 17, 2004 PUI Rules Team)
And also another official ruling from The Rulings Compendium LVX:
Silver Wind (Mothim - OP Series 7; Mothim - Secret Wonders; Dustox ex - Legend Maker; Others)
Q. If I use Silver Wind once, then I use it again on my next turn, it would do 60 damage (assuming there's no weakness or resistance, and that neither active Pokémon is switched out). On the third turn would it do +30 more damage (bringing it up to 90), and on the fourth 120 damage, and so on?
A. Sorry, but Silver Wind only adds 30 to the base damage done by the attack; it is not cumulative in the manner you suggest. (Jun 17, 2004 PUI Rules Team)
Joe pointed out in comment that there is also another official ruling in the latest version of compendium, The Rulings Compendium Black and White:
Meteor Mash (Metagross - Celestial Storm)
Q. If I use Metagross' "Meteor Mash" attack several turns in succession, does the damage keep piling up as 60, 120, 180, 240, etc.?
A. No, Meteor Mash tops out at 120. It only adds +60 to the base 60 damage, it does not accumulate on successive turns. (Celestial Storm FAQ; Aug 2, 2018 TPCi Rules Team)