NorthernTel doesn't have Conditional Call Forwarding (forwards calls only when they are unanswered or the phone line is busy). However, you can configure a simultaneous ring group with NorthernTel that achieves the same thing.
Add Numa to the SimRing in NorthernTel
Reach out to NorthernTel and indicate that you want to add a SimRing to your account (if you don't already have it). There may be an additional charge for this.
Have them add your business's Numa number to the SimRing. You can find your Numa number by going to Settings > Phone numbers.
Go to Settings > Phone numbers > Select your Numa number > Advanced Options. Add an artificial delay. Choose a delay time that is shorter enough so that you have time to answer the phones that ring in the group but not too long (or else a non-Numa voicemail may kick in).
How to Confirm It's Working
You can place a test call by going to Settings > Phone numbers > Test my setup or call your business's phone number from your personal phone. If nobody answers, you should hear your Numa Voicemail greeting.
Troubleshooting
No matter what, don't hesitate to contact Numa support! We'll happily work with you and your service provider to get you all set up.