Quando o botão de ligar não funciona, eu não posso ligar o celular.

It’s the same verb (ligar), but it’s used with two different meanings:

• botão de ligar = the power button (literally “button for turning on”)
• ligar o celular = to turn the phone on
  In Brazil, ligar can mean to turn on a device, and also to call someone. Context (and the object) tells you which meaning it has.

Literally, botão de ligar means “button to turn on.” It’s natural and common. You may also see:

• botão liga/desliga = “on/off button”
• botão de energia = “power button” (also common, slightly more “technical” sounding)

Both can work, but they’re used differently:

• botão de + infinitive is a very common way to label a control by its function: botão de ligar, botão de abrir, botão de fechar.
• botão para + infinitive emphasizes purpose more explicitly: botão para ligar (“button in order to turn on”).
  On devices and in everyday speech, botão de ligar is especially common.

In Portuguese, ligar (turn on) is usually transitive with devices: ligar o celular, ligar a TV, ligar o computador. You don’t need a preposition.
You can use ligar em in other meanings, especially:

• ligar em alguém = to pay attention to someone / care about someone (regional/colloquial)
• ligar para alguém = to call someone

Portuguese often uses the present for general truths or repeated situations:

• Quando X acontece, eu não posso Y.
  If you wanted to make it specifically future, you could say:
• Quando o botão de ligar não funcionar, eu não vou poder ligar o celular. (future situation)

Both are possible, with a nuance:

• não funciona = doesn’t work (general fact / it fails)
• não está funcionando = isn’t working (right now / at the moment)
  So Quando o botão de ligar não funciona sounds like “When the button fails/doesn’t work,” while não está funcionando highlights a current malfunction.

Yes, often. Portuguese frequently omits subject pronouns because the verb ending shows who it is:

• Quando o botão de ligar não funciona, não posso ligar o celular.
  Including eu adds emphasis or clarity, but it’s not required.

Portuguese commonly uses the definite article (o/a) where English might use a possessive. It’s natural to say:

• ligar o celular = turn on the phone (often understood as “my phone” from context)
  If you want to emphasize ownership or contrast, use:
• ligar meu celular = turn on my phone

Here de is marking a relationship like “of/for” describing the button’s function. It’s similar to English patterns like “start button,” “reset button,” etc., except Portuguese often uses de + infinitive:

• botão de start (loanword, less common)
• botão de reiniciar (restart button)
• botão de ligar (power button)

Yes, with a slightly different feel:

• Quando o botão de ligar não funciona... = when/whenever this happens (more like a recurring situation)
• Se o botão de ligar não funciona... = if it doesn’t work (more conditional/hypothetical)
  Both are acceptable depending on what you mean.

It’s recommended. When the sentence starts with a dependent clause introduced by quando, you typically use a comma before the main clause:

• Quando ..., eu ...
  In casual writing you might see it omitted, but the comma makes the structure clearer.

Usually no, because ligar meaning “to call” normally uses a person/number or para:

• ligar para alguém = to call someone
  And “call the cellphone” would more likely be:
• ligar para o celular (dele/dela) or ligar no celular (common in Brazil)
  With botão de ligar in the same sentence, the “turn on” meaning is strongly signaled.

Some natural alternatives:

• Quando o botão de ligar não funciona, não dá pra ligar o celular. (very common: não dá pra = “it’s not possible”)
• Se o botão de power não funciona, não consigo ligar o celular. (não consigo = “I can’t manage to”)