Why does my iPhone ring just once before going to voicemail?
My iPhone rings just one time before going to voicemail. If I turn off the wifi, the phonecalls work fine.
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I found the answer to this question which has been posted many times without an answer since at least 2017. This is what worked for me. I have an iPad and an iPhone and it seems I configured my calls to be routed to my iPad. This wasn't mentioned in any of the other answers on this page. I even went so far as to reset my network settings. Hopefully, this saves other iPhone users a headache.
Go to:
Settings - Phone - Calls on other devices - Turn this off.