If you don't have a phone, you are allowed to ask a person outside the game who's there. However, that person isn't allowed to look at your game. Texting is also fine, but no matter how they are contacted, the person needs to respond in a timely manner. (And if your friends are anything like mine, I would not let my success depend on them responding promptly to my text messages.)

Whichever way you "phone" a friend should be open to all players so their choice can be verified. Use speakerphone, show everyone the text, etc.

You are allowed to read each mode in full. The letters are there to make it more like a game show.

"If they don't answer" means you failed to contact anyone or the person you contacted didn't make a choice in a reasonable amount of time.

If they choose A or B, and there are no legal targets available, ask them to choose another mode. You can't cast Phone a Friend with an illegal mode chosen. Yes, you could just eliminate modes that can't be chosen from the beginning, but what fun is that? If an opponent ends up choosing the mode, they also must choose a legal mode.

You can set up the general context of the call ("I'm in a Magic game and I need someone outside the game to make a choice"), but you shouldn't explain anything that's going on in the game or what the consequences of their choice may be.

The friend you phone can be someone who doesn't play Magic. If so, inform them that they can learn to play at a local game store near them (refer to rule 100.6b in the Magic Comprehensive Rules for details), or they can head to MTGArena.com and download today!