How to extend ringing time to 30 secs on my iPhone 15 with iOS 18?

you are 100% wrong - the ios18 did totally stuff it up - but yes the carrier controls the ring duration but the update automatically applied live voicemail which limits the rings and sends a message instead. So to get the long 30 second ring back - you have to go to Settings scroll right to the bottom and tap Apps then to Phone then scroll down to Live Voice Mail and turn it off. If you are in Oz and use Telstra you have to use the **61*101**30# then press call - wait and then you get screen messages to say its updated and which you can then dismiss - which is what i had done ages ago and just done again to make sure !!!

ps Telstra dont allow more than 30 seconds it seems as trying to say 45 or 60 instead of 30 got you nowhere!