The voice on the phone and Carol are not the same person.
(Deltarune Chapter 4 + Weird Route spoilers.)
EDIT: I have since developed and adapted this post into a theory video. Find link to it at the bottom of the post.
I've heard people say "the voice is closer in the Weird Route ending so that's why it sounds different" but it's not. The voice on the phone (which I'm assuming is the Knight) sounds different then Caorl's Weird Route voice mail, when both scenes have Kris right next to the phone.
The only difference is in the Holiday Mansion phone call we're in Kris's other hand. Meanwhile, the were in the cage in the weird route voice mail. If anything the ending phone call in the mansion should sound clearer.
So why does the phone voice say "I'll be right there."
It's talking about the Dark Fountain. Earlier, it says the Dark Fountain would be in the church "tonight," but after the mansion hangout happens, there's a dark fountain in the afternoon.
It's clear that Susie getting the guitar bumped up the Knight's plans, so they decided to create the church fountain earlier.
This goes into speculation but it might explain why they stay in the church. Because maybe the Knight cannot stay in the light world for prolonged periods of time, and less so at day time. It didn't have time to set up a head Darkner and everything and leave. (Which explains y'know, the titan summoning at the end.)
As far as the Knight knew, Susie likely had the code to the bunker would be able to break in after this fountain. (She didn't as she didn't get to write down the code though.) Even if you need all three codes to break in, it would still mean Susie was one step closer as far as the Knight knew.
So, this fountain is a last ditch attempt to maybe start the roaring by summoning a Titan. This reeks of an act of desperation.
Then why does Carol show up right away?
Asgore tipped her off.
Think about it? We already know that Asgore is Carol's gardener. And we already know there's something really suspicious going on with Asgore. (Which is presumably his search for Dess.)
This goes into "Hometown Conspiracy/Hot Fuzz cult" theorizing, but seeing as Asgore's allowed into Carol's room, (plus all the conspiracy board stuff that's likely Dess related,) he's likely in on whatever conspiracy is going on. If Dess's disappearance is related to the bunker, he likely knows one of the codes is hidden on Dess's guitar.
So, here's the chain of events I see happening...
Kris calls the Knight to warn them that Susie has the guitar. In response they bump up their plans and make the Chapter 4 dark fountain early.
Shortly afterwards, Asgore overhears Susie playing the guitar.
Asgore quickly calls Carol to tell her someone has December's guitar and they might learn the bunker code. She immediately leaves the town hall.
Once Asgore sees Carol's car arrive, he goes to check who's playing the guitar, with the snack trail as a convenient excuse.
Carol arrives, and takes the guitar.
The fact that Asgore pauses eating at the mention of "a code" is extra suspicious. Almost like that confirms his suspicion that Susie and Kris were looking for the bunker codes.
It all adds up together. Carol may still be working with the Knight but I don't think she's directly the one on the phone call. Chapter 4's prophecies have already suggested that the Chapter 5 Dark World would be flower king. It makes sense if Asgore's interfering with our Bunker-Code search if it's connected to whatever he plans to do in Chapter 5.
Anyway that's why the Roaring Knight is Dess and not Carol. Have a good day.
EDIT: Here's the link to the video adaptation of this post!