Unable to change the number of rings before my voicemail picks up on iPhone 13 Pro Max

Contact your carrier as they are the ones that determine and can change how many rings before your iPhone calls go to voicemail. But note that it is not by rings but seconds, and even if they can change it, it probably won’t be much.

phone time to voicemail

https://en.m.wikipedia.org/wiki/Call_forwarding

If i it rings once then goes to voicemail, then it is timing out.

Google how your particular provider increases the ring time,

or try dialling this code 

**61*vvv*11*XX#

where vvv is voicemail number

XX is the seconds of ring time you want, up to 29 max