Toplantıdayken telefonumu sessize alıyorum.

It means while at the meeting. It’s built as:

• toplantı = meeting
• -da = locative case (at/in), choosing -da rather than -de by vowel harmony
• -yken = while (the buffer y is used because the preceding part ends with a vowel)

So: toplantı + da + yken → Toplantıdayken = while at the meeting.

Because -yken after a noun typically attaches to a case-marked form to mean while in/at/as. You want while at the meeting, so you add the locative -da first:

• Toplantıdayken = while at the meeting
• Toplantıyken would suggest while (it is) a meeting, which is not idiomatic here.

Compare: Evdeyken (while at home), not Evyken.

• -yken = while, during (simultaneous actions or states). Example meaning: While I’m in a meeting, I silence my phone.
• -ınca/-ince = when/once/as soon as (a triggering moment). Example: Toplantı başlayınca telefonumu sessize alırım = When the meeting starts, I put my phone on silent.

Use -yken for overlap in time; use -ınca/-ince for when X happens, then Y.

Step by step:

• telefon = phone
• • -(I)m (1st person possessive) → telefonum (my phone). The vowel becomes u by 4-way harmony because the last vowel in telefon is o.
• • -(y)I (accusative) → telefonumu. The last vowel before the case is u, so you add -u; no buffer y is needed because the word ends in a consonant.

Sessize almak literally means to take (something) to silent, i.e., to put it on silent. The -e is the dative (to/toward) used with adjectives/nouns to express changing something into a state:

• sessiz (silent) → sessiz-e (to silent [mode/state]) Similar patterns:
• Uçak moduna almak (to put into airplane mode)
• Sessize çevirmek also exists but sessize almak is the most common for phones.

Both are possible, with a nuance:

• Alıyorum (present continuous) can describe current, ongoing action or a repeated habit in conversational style.
• Alırım (aorist) is the default for habits and routines. So for a general habit, many speakers prefer Toplantıdayken telefonumu sessize alırım, but alıyorum is also common and natural in everyday speech.

Yes. Adverbials and objects are flexible, but the verb typically comes last. Some natural variants:

• Toplantıdayken telefonumu sessize alıyorum (neutral)
• Telefonumu toplantıdayken sessize alıyorum (slight focus on my phone)
• Toplantıdayken sessize alıyorum telefonumu (more marked; stylistic/emphatic)

Keep the verb final for the most neutral tone.

Locative case alternates by harmony and voicing:

• -da/-de by vowel harmony (a/ı/o/u → -da; e/i/ö/ü → -de)
• It also has -ta/-te after voiceless consonants Since toplantı ends with a vowel and has back vowels, you choose -da, yielding toplantıda. No devoicing to -ta is needed because the preceding sound is a vowel.

Use the plural and optionally keep -yken:

• Toplantılarda telefonumu sessize alırım = I silence my phone at meetings.
• Toplantılardayken telefonumu sessize alırım = While I’m at meetings, I silence my phone. Both are natural; the -yken version highlights simultaneity a bit more.