How can I repeat my ringtone and increase the delay until a caller is sent to voicemail? I have a custom ringtone (30 seconds in length), but it goes to voicemail after approximately 20 seconds. Can I repeat the ringtone multiple times before redirecting the caller to voicemail?
You need to talk to your phone service provider.  They can set the "delay before divert to voicemail", or provide you with a magic number to dial so you can set the delay.

I know my provider only allows a maximum of 25 seconds, which I agree is too short.