my calls are ringing once then going straight to voicemail

Ask your network provider to increase ring time

or

Google for the dial-in codes to increase the ring time for YOUR carrier

example for O2 carrier –

 **61*901*11*20#

will increase ting time to 20 sec

My iPhone 13 Pro Max has recently started doing this also! Sometimes it never rings at all before going to VM and sometimes it rings once and goes straight to VM. I have checked all the typical things that would cause this; Do Not Disturb, Airplane Mode, Updates, Call Forwarding, Silence Unknown Callers, Carrier Update & Ringer Volume all have the appropriate settings. I restarted my phone which worked for less than 24 hours and it started doing it again! What is going on? I am missing calls and am constantly having to look at my screen to see if I have missed calls.

See the Apple Recommended answer. Its what I do.