alt Hacker NewsVoice "circuits" are largely a thing of the past. Most cellular calls are VoLTE or VoIP directly (though I'm certain there's shaping/QOS/etc., in play).
On the spam topic though, it is, or was, a common tactic for some spammers to do just enough on the call initialization front to generate a missed call or similar without actually ringing a device, leave no VM, the goal being to prompt you to return the call. Though this doesn't fit with the time delay aspect.
They are still separated from the regular traffic (via VLAN/VxLANs/MPLS, whatever). So your device will have a separate hidden IP (IPv6 or IPv4) that is used ONLY for voice connections, and it's typically firewalled to hell and back.
One significant reason is roaming and inter-provider calls. Internal calls can go through a fully IP-native network, but external calls often need SS7 singalling and arcane interconnects. And you really don't want your SS7 switches to be even pingable from the public network.
BTW, one thing to check: misbehaving WiFi calling. It works by establishing an IPSec connection with the ISP's infrastructure for that hidden IP. It should not be possible on a cellular connection (because what's the point?), but given the decline in technical expertise, I'd also disable it to see if it helps.