Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Are you sure the problem is with your phone system? if it is set to 2 rings, it should not - *if* the voicemail is indeed located in the NEC SV8100 system - pick up voicemail on the second ring's verge.
If the voicemail is provided by your phone co., then it might need to be set to after-three-rings.
If the voicemail is on your phone system, locally to you, for sure, then let me know, and I will look up the matter of the setting and advise you (if I can find the exact information, and if you have the voicemail administrator or phone system administrator's password ***** the NEC system.) Let me know, please. Thanks.