Question: Without warning, a wizard casts *telepathy* targeting a friend who is on a different continent.

> 
> You create a telepathic link between yourself and a willing creature with which you are familiar.
> 
> 
> 

How is it decided whether the target is willing? Does the wizard get 6 seconds to quickly introduce herself? Does the target "feel" a "friend request/phone call" and envision the caster in their head and then decide whether to let them in/answer?

I wondered if the spell assumes the target is willing until the target decides otherwise, but there is no option for the target to end the spell.

Answer: 
### Short answer, you will need to establish an agreement ahead of time. As the check occurs before the connection is ever established.

This is apparently not a spell used for cold calling.

As explained by @Fie (emphasis theirs)

> 
> according to the spell targetting rules in XGE (pp. 85–6), the validity of the target (i.e. whether the creature is willing) must be determined *before* the spell has any effect (including the target recognising you as the creature it ***is* communicating** with).
> 
> 
> 

Meaning that though you are familiar with the target, they must be willing to receive your message before they receive it. A short conversation among friends can be had, "Hey, is it ok if I use telepathy to communicate with you in the future?" to establish that willingness in some regard ahead of time.

If the target for some reason becomes unwilling, the spell will fail without contact ever being established and the target not realizing you attempted to reach them. Perhaps they are busy at the moment, but that doesn't stop them from later being willing once more.

**The Willingness check will occur before connection is made on casting the spell.**

Alternatively, there may be a discussion on the hypothetical,

* "if A somehow knew that it was B calling, would they accept the call?" willingness being applied that way, as they are willing in this case.

But it is unclear if the question instead is,

* "are they willing to accept an unknown call before knowing who sent the request?" which might not be the same willingness.

Could be stated more clearly in either case.

Curious though, the duration *is 24 hours* without an off switch *other than* moving to another plane of existence. Once the spell succeeds, the willingness of the creature to continue the link is seemingly **no longer** considered.
The rules don't specify the exact mechanism, so the GM will need to adjudicate.

If the target of the spell is a PC, the GM will need to give the player enough information to decide if the player is willing.

If the target of the spell is an NPC, the GM will need to decide for the NPC.

The GM may want to establish house rules for exactly how it works, but the need for those rules is going to vary tremendously from table to table.

In many cases, the adjudication should be pretty easy.

Let's assume the target is a PC:

GM: You get a strange feeling. Your friend, Wendy the Wizard, is trying to establish a telepathic link with you. Are you willing?
Player: I...how do I know it's Wendy?
GM: You just know.
Player: How do I know?
GM: It's magic, but it's sort of like seeing someone's face or hearing their voice, you recognize them.

At this point, the player decides whether to accept the connection or not, or maybe they ask more questions.

A given table may need to have further discussion, or may not.