VA's Duty to Assist assists you in getting evidence, such as medical records, to help support your case and only applies during your initial claim and if you file a Supplemental Claim.
Are you going to request  Higher-Level Review or Board Appeal?
Yes,  am going to request a Higher-Level Review.
Your Duty to Assist will not apply; but the reviewer or judge will make sure the VA met its Duty to Assist during your initial claim.
I'd also like to learn more about the Higher-Level Review. Is there a spot on your website where you keep a list of frequently asked questions?
Yes there is, Go to FAQs and there you can get answers to some of your questions
Can you please tell me on what should I expect if I request a call with a senior reviewer regarding my case?
Yes, you should expect a call at the phone number you gave them or the one your representative gave them on the Higher-Level Review form in which they will schedule a time to discuss your case with you.
what can happen no one answers calls from the top reviewer?
in that case they will leave a voicemail.
What if i disagree with a Board decision?
Your decision comes with the information you need to address errors.
What happens if I miss the deadline for submitting my request?
It is in your best interest to submit your request by the deadline to receive the maximum benefit if your case is granted.
What does the new and relevant evidence help with?