Many people, myself included, has thought that this person on the phone with Kris is Carol Holiday.
However, I no longer believe so.
Let's break down the reasons, one by one.
"But, she was home early! After Kris called about Susie getting the guitar!"
This is one of the most common arguments for Carol Caller. Indeed, it all seems to line up: Carol wasn't going to be home for hours, until the call from Kris sent her rushing back. And her first priority upon entering was to take the guitar away from Susie.
But, is it really that simple?
What if Carol came back for something else entirely, and it had nothing to do with Kris?
Exhibit A: the same segment in Weird Route
On Weird Route, Kris never made the call. And yet, Carol still came home early, asking everyone to leave. Why would she had come if Kris didn't call her?
"How do we know if Kris didn't call offscreen?"
Because on Weird Route, Kris didn't know Susie got to the guitar until later, when they were both outside.
And from Susie's account, we can gleam that Susie went from Dess' room—found guitar—went to living room with guitar, and was held up by Asgore until Carol's arrival.
Meanwhile, we see Kris went from Noelle's room—bathroom, and had a massive crashout upon entering. Said crashout was also interrupted soon after by the news that Carol came home.
If the call did somehow happen, it would leave an extremely small window of time for Carol to come home. She would probably need literal wings for that.
And that's without considering how dissonant and improbable it would be for Kris to notice Susie with the guitar and make a call for it in enormous emotional anguish and rage.
Therefore, I think it's unreasonable to assume that Kris still made the call on Weird Route.
"What if Carol was summoned by Kris on Normal Route, but by someone else on Weird Route?"
That's essentially acknowledging that someone else could have called Carol home. What stops them from also doing so on Normal Route?
So if Kris didn't call Carol home, who did?
He was, after all, the one who revealed that Carol was coming home early.
We also know that he'd been working with Carol to find out what happened to Dess.
And, on that day, he also happened to have an reason to call Carol.
To show her his latest discovery. A black shard.
Exhibit B: the call from actual Carol at the end of Weird Route Ch.4
And as you can see, her speech was completely normal. Not like the Mysterious Caller's voice at all.
These calls happened at around the same hour in late night. Carol would be in the same place, calling with the same device. If the Caller were Carol, it makes no sense for her voice to be different.
"What if she changed her voice to sound dark and ominous?"
Okay, but why would Carol need to do that? Kris had her contact. They knew who she was either way.
"To obscure her identity from the SOUL? The Caller even knows about Kris' SOUL situation!"
The Caller never framed the SOUL as a threat, though. They even said that Kris cannot enter the dark world without it. And truthfully, the SOUL does not have the sentience to be dangerous on its own; the Player does. And nothing suggests that the Caller knows about the Player.
And, while we're on the topic...
The Caller knew about dark fountains and dark worlds. I highly doubt that Carol knew those things, because if she did, then she was already extremely close to finding her daughter (on the premise that Dess is the Knight). Asgore and her wouldn't need evidence boards like they were still crafting theories, they could just make their own fountains.
So no, I don't think Carol could be the Caller. Which, would also mean that she hadn't been manipulating Kris and holding them to a promise.
"Then, what's with her 'you are always welcome here' to Kris? Kris was even uncomfortable, too!"
I have speculated that Kris saw Dess last.
To Carol, this meant that Kris Dreemurr remained the only person who could provide reliable clues to finding Dess. She was hoping Kris would one day open up, and being nice to them means it'd be more likely to happen.
Kris wasn't comfortable, of course, because the Dessappearance was a deeply traumatizing event that they still feel guilty for. They weren't ready to disclose anything yet.
But, we know that they aren't being harmed by the biggest authority figure in town.