Minun puhelimeni on äänettömällä.

Yes. Both mark possession.

• minun = my
• puhelimeni = my phone because -ni is the 1st person possessive suffix

So minun puhelimeni is literally something like my phone of mine. That sounds redundant in English, but it is normal in standard Finnish.

In practice:

• puhelimeni = correct, standard
• minun puhelimeni = also correct, often a bit more explicit or formal
• mun puhelin = very common in spoken Finnish
• minun puhelin = not standard written Finnish

Yes. That is completely correct.

Puhelimeni on äänettömällä means the same thing: My phone is on silent.

Finnish often allows possession to be shown just with the possessive suffix:

• puhelimeni = my phone
• kirjani = my book
• ystäväni = my friend

So in this sentence, minun is optional if the suffix -ni is already there.

A word-by-word breakdown is:

• minun = my
• puhelimeni = my phone
• on = is
• äänettömällä = on silent / in silent mode

A more literal sense of the whole sentence is:

My phone is in silent mode.

Äänettömällä is in the adessive case, which usually has the ending -lla / -llä.

The adessive often means things like:

• on / at / in a state or setting
• using
• at a place

In this sentence, it is used for a state or mode. Finnish often uses the adessive with devices and settings:

• Puhelin on äänettömällä. = The phone is on silent.
• Puhelin on värinällä. = The phone is on vibrate.
• Tietokone on päällä. = The computer is on.

So äänettömällä does not mean just silent as a plain adjective here. It means in silent mode.

Because the sentence is not saying that the phone is a silent phone as a basic description. It is saying that the phone is in the silent setting.

Compare the difference:

• äänetön = silent, soundless
• äänettömällä = on silent, in silent mode

So Finnish uses the adessive form here because this is about a current mode or setting, not just a permanent quality.

The basic form is äänetön.

That adjective means silent, soundless, or mute depending on context.

Its forms change like this:

• äänetön = nominative
• äänettömän = genitive
• äänettömällä = adessive

This type of word changes its stem when declined, so the form is not built by simply adding -llä to äänetön.

Because adjectives ending in -ton / -tön have a special stem in other cases.

For example:

• äänetön → stem äänettömä-
• then add the adessive ending -llä
• result: äänettömällä

This is a normal pattern in Finnish. Similar examples:

• mahdoton → mahdottomalla
• onneton → onnettomalla

So this kind of change is something you simply learn as part of the declension pattern.

Yes. On is the 3rd person singular of olla, which means to be.

So:

• Minun puhelimeni on äänettömällä.
• literally: My phone is on silent / is in silent mode.

This is a very common Finnish structure:

[thing] + on + [state/mode]

Minun puhelimeni on äänettömällä. is grammatically correct standard Finnish, but it sounds a bit more careful or formal than everyday speech.

In casual spoken Finnish, you are more likely to hear:

• Mun puhelin on äänettömällä.
• Mun kännykkä on äänettömällä.

Notes:

• mun = spoken form of minun
• kännykkä = cellphone / mobile phone, very common in speech

So the original sentence is good standard Finnish, but not the most casual everyday version.

Yes, but the original order is the most neutral one.

• Minun puhelimeni on äänettömällä. = neutral
• Puhelimeni on äänettömällä. = also neutral and very natural

Finnish word order is flexible, but changing it often changes emphasis. For example, if you move words around, you may sound more contrastive or marked.

For a learner, the safest natural choices are:

• Puhelimeni on äänettömällä.
• Minun puhelimeni on äänettömällä.

Yes. This is a very useful pattern in Finnish.

Examples:

• Puhelin on äänettömällä. = The phone is on silent.
• Puhelin on värinällä. = The phone is on vibrate.
• Radio on päällä. = The radio is on.
• Tietokone on pois päältä. = The computer is off.
• Ovi on lukossa. = The door is locked.

These expressions often describe a state, condition, or setting, and Finnish frequently uses special case forms for that instead of a simple adjective.

They probably would, but it is not standard Finnish.

Why?

Because in standard Finnish, if you use minun before a noun, you normally also use the possessive suffix:

• minun puhelimeni = standard

In spoken language, people often drop the suffix and use a spoken pronoun instead:

• mun puhelin = common spoken Finnish

So for a learner, the best choices are:

• minun puhelimeni = standard
• puhelimeni = standard
• mun puhelin = spoken

But not usually minun puhelin in careful Finnish.