Does a Trumpet card count as following suit?
In most games with a trump suit, you must follow with a card of the suit that has been led, unless you are void in that suit (have no cards of that suit), at which point you may play a trump card. Given that, I would assume the rules in Trumpet are the same, and you cannot play a Trumpet unless you're void in the suit led.

This thread on BoardGameGeek agrees, so I'd say it's fairly common to play by that rule, even if the original rules are unclear.