How do phone apps like "Repeater Phone" actually work?

I have some friends with iPhones that are connecting in with an app called “Repeater Phone”.
They just plug in their Allstarlink.org main admin account credentials and it “just works”.
I do not have an iPhone myself to test or understand this.
How does it actually gain access to nodes like it does?
Also it shows up in the Allstar network as “CALLSIGN” and not a node number.
It does not appear to be using IAX2 directly like any other node.
I can only assume this is using some kind of cloud broker provided by the app provider or something and conneecting on it’s behalf?
Or is there another path into the allstar network I’m not familiar with?
And how is it that are they presenting as a callsign instead of a node number?
I would expect any app like this would normally need to register as a “normal node” and follow those rules, but this appears to be something different and users are not setting these up as a normal node… They are simply plugging in their main allstarlink.org creds.
Any insight into this would be very much appreciated.
Thanks!!
-Steve

Short version:
RepeaterPhone for iOS, Transceive for Mac OS, and DVSwitch Mobile for Android, as one of the available modes, uses Web Transceiver. This is still IAX2, but a different context.
This used to be a thing one could use to call nodes directly from the website using java, but this has been deprecated.
Using this method, you are issued a token by allstarlink.org when you login. An unauthenticated call is placed to a node on the allstar-public context, which has as part of it’s dialplan, a mechanism for verifying that the token for the incoming CallerID (the callsign) is valid. If it is, it lets the call through. If it can’t validate, then it hangs up.
This doesn’t require a node number, as the call isn’t being directed to radio-secure, just an account on allstarlink.org.
There are downsides to this, so not all node owners, particularly some larger networks, allow allstar-public on their nodes. This can be disabled by commenting or deleting the entire allstar-public context in extensions.conf, or it can be handled more gracefully in one of a few different ways.
I have a relatively large, multi-mode/multi-node system, on which I have disabled it on all nodes but one, which makes it easier to manage and isolate when problems do occur.
Repeaterphone does optionally allow you to authenticate to a node using IAX2 credentials as well, but not directly to allstarlink.org as a stand-alone node. You need another node through which to connect.

DVSwitch Mobile does have a thing called node mode, which registers as it’s own node number.

Thank You!!
Extremely helpful and I learned something new I didn’t really know anything about!