Скажи, действует ли гарантия на мой телефон?

Скажи is informal (to a friend, someone you address as ты). For polite/formal speech, use:

• Скажите, действует ли гарантия на мой телефон? (to вы) You can also add пожалуйста: Скажите, пожалуйста, …

ли turns the clause into a yes/no question, similar to English whether/if in an embedded-question style. It typically comes right after the word being “questioned” or focused—here the verb:

• действует ли гарантия… = is the warranty valid/does it apply…? Placing ли after действует is the most neutral, standard pattern.

Sometimes yes, but it changes emphasis and can sound less natural depending on placement. Neutral:

• Действует ли гарантия на мой телефон? Possible with different emphasis:
• Гарантия ли действует на мой телефон? (odd/rare; sounds like you’re contrasting “warranty” with something else) In most everyday cases, keep ли after the verb: действует ли.

For a warranty, Russian commonly uses действовать meaning to be valid / to be in effect / to apply:

• гарантия действует = the warranty is valid Работает is usually for devices, systems, or services “working” physically/operationally, not for legal validity.

The grammatical subject is гарантия (nominative singular feminine). The verb agrees with it:

• гарантия (she/it) → действует телефон is part of a prepositional phrase (на мой телефон) describing what the warranty applies to.

After на in the meaning for / covering / applying to, Russian uses the accusative case:

• на телефон (accusative) For inanimate masculine nouns like телефон, accusative looks the same as nominative, so телефон doesn’t change form. The pronoun does show accusative:
• мой (nom) → мой (acc masc inanimate; same form)

Yes. на телефон is fine if context already makes it clear whose phone. Adding мой makes it explicit: on my phone / for my phone. In a shop or service center, both are common:

• …на телефон? (more general)
• …на мой телефон? (more specific)

That word order is generally awkward. The most natural patterns are:

• Скажи, действует ли гарантия на мой телефон?
• Скажи, на мой телефон действует ли гарантия? (possible but marked; emphasizes на мой телефон) Most of the time, stick with действует ли гарантия… as the neutral order.

Yes. You can ask directly:

• Действует ли гарантия на мой телефон? Or even more conversationally:
• Гарантия на мой телефон ещё действует? (intonation question; no ли)

With ли, it’s a clear yes/no question regardless of intonation:

• Действует ли гарантия…? (very explicit) Without ли, Russian relies more on rising intonation and context:
• Гарантия на мой телефон действует? (more conversational; can sound like “It’s still under warranty, right?”)

Add ещё:

• Скажи, действует ли ещё гарантия на мой телефон? Commonly ещё goes near the verb, but several positions work:
• …ещё действует гарантия…
• …гарантия ещё действует…