Learn Spanish (Latin America): Mi teléfono está muerto.

Yes! It comes from the verb morir (to die). Normally, an -ir verb would end in -ido to form the past participle, but morir is irregular. It becomes muerto, never "morido".

Why use está instead of es? Isn't being dead a permanent state?

This is a famous exception that trips up many learners! In Spanish, we use estar for states that are the result of an action. The phone died, and its resulting state is "dead". Therefore, you always use estar with muerto.

If muerto is a past participle, why can its ending change?

When past participles are used as adjectives with the verb estar (like in this sentence), they have to match the noun's gender and number. Since teléfono is masculine and singular, we use muerto. If you were talking about your battery (la batería), you would say está muerta.

Do Spanish speakers actually say their phones are "dead" like we do in English?

Yes! Just like in English, saying Mi teléfono está muerto is a perfectly natural way to say your phone's battery has run out. You might also hear people say Mi teléfono está descargado (My phone is discharged).