Phonics Facts: -ed sound

Why do some words that have -ed make the "t" sound?

/ed/ sound is made as a separate syllable sound when it comes after words ending in d or t ( rested, dreaded)

- /d/ sound is made when it comes after voices consonants (spilled, buzzed)

- /t/ sound is made when it comes after unvoiced consonants (rushed, stumped)