Would someone who is mute be able to return a message? I know that the casting requires a verbal component, I would imagine sending a message back would also require speaking out loud even though you hear the message in your mind.

in my game i usually make it so that they have to be willing

Can you use it to send a message to someone that you don't know but was described to you by someone else?

the fact that artificers don't get sending is a travesty. (they get sending stones, why not the sending spell?)

For me as a DM it works like this: Familiarity means you have to have met this person. Thus your magic subconcious can take in the astral signature of the person you want to reach (this eliminates abuse like "I know my god / my patron / my favourite angel / the king / the BBEG, etc.).

Therefore if someones uses Disguise Self or Alter Self to impersonate another person the sending gets to the impersonator (as the astral signature is not altered by these spells).

Furthermore the spell works like a "fire and forget" message. You send the message and wait for a reply, not knowing if the receipient did get the message and does not want to answer, is sleeping, is dead or whatever. More like an email to an magical / astral signature than a telepathic bonding.

If I like I can add something like "The astral signature slightly changes over the years as your soul develops with your experiences." to prevent my players contacting NPCs they have met once decades ago.

Sending can target creatures on other planes. Presumably, you could contact the afterlives.

Sending is one of those spells which I really wish were never added to D&D. (Along with the ones which allow you to just unceremoniously bring someone back from the dead of course, but that's beside the point.) It's not that it's overpowered or otherwise imbalanced, but the mere potential to just ask anything from anyone no matter where they are seems like it could seriously mess up a perfectly good story scarily quickly.

I'd rather that something like this be restricted exclusively to Sending Stones or similar objects, where to contact someone so easily like this you'd need to give them a physical object, something which your DM has much more control over than some ephemeral spell effect.

There is a verbal component of the spell, I would assume the caster needs to speak out loud their message despite the recipient hearing it in their mind.

There should be a 4th level spell called Receiving, which allows you to eavesdrop on any Sending (send or reply) if either participant is within 1 mile of you. The Sending spell you are eavesdropping must have been cast within the last three rounds. If you eavesdrop on the initial message, you will also eavesdrop on the reply (if any).

We have two sending stones in our party.  Finnan takes one, and Mythoras takes the other.

To NPC 1, as it's not so much about knowing the identity of the person, as it is about having a connection with another's mind.

I would think of it like this:  Barry is impersonating Leonard. He meets Rachel, who asks for his number. Because Barry is impersonating Leonard, he gives Leonard's number to Rachel. Thus, when Rachel tries to contact the guy she met, she contacts Leonard, not Barry.

Edit: upon reading more comments, I think it could go both ways, and both are interesting. I would base who is contacted on how well the impersonation is done.