alt Hacker NewsAre you sure you don't have any misbehaving spam detectors? The voice circuits are VERY robust, low bandwidth, and cellular providers prioritize voice calls. There's just no reason to purposefully throttle the voice calls, the bandwidth savings would be negligible.
But it's possible that your cellular provider has misconfigured STIR/SHAKEN validation and/or some badly behaving spam call detection.
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Voice "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.
Google is currently carrying out a "call screening" experiment on Pixel phone "owners" (mentioned, but cannot be disabled in the app settings). OP is specifically mentioning iphone, though.