What counts under different warranties?
The standard warranty only covers things that are technical problems with the phones themselves. Always ask how a problem happened - if a problem happened because the customer dropped their phone, it is not covered under standard warranty. Even if they dropped it just a little. Even if it fell on carpet. Extended warranty, at least in late-game, does not cover full phone replacements - generally upsell those when possible. No warranty covers nothing. A VIP warranty with an incorrect code is a no warranty.

Quick solutions
The "garbled image" or corruption can only be fixed by a factory reset. When it says that, propose that first, then either repair or upsell if needed.
"Drop-error" guy (Jimmy) can be immediately handled by saying you can't help. There is no penalty for this.
The old guy - as pointed out by BearBear - can be handled by saying yes and asking what the problem is.
The person speaking in English and Russian broke his screen. Just explain that he needs to replace his screen and he'll go away.

The terminal code is next, play, play, previous, next, previous, play, previous, next.

How do I access BIOS?
BIOS are located in the boot-up menu.

How do you handle the Melville chat?
You need to ask for his customer number - the two-factor won't match, but that doesn't seem to matter. Once you open his profile, you will have the option to small-talk about spying on customers. Click that, then reply yes when he asks if you agree with his philosophy.

How do you know you're on the right path for the Corporate Weapon ending?
Instead of the usual Indigo Fox message, you'll get a message from [spoiler[Jessyca Peiper[/spoiler] talking about some issue. Just send their GPS to Melville.

How do you handle the guy who threatens your family?
You need to unlock the phone he tells you to and hit the call button in the bottom-left.

What counts as "upgrades"?
Everything in the extras menu counts. Including cat faces.

Any further questions feel free to comment! I noticed a lack of comprehensive guides in this community so I'm looking to write one - drop any questions and I'll add them to the guide!