(360) 637-6999 is a Prison Prepaid Call
You can block the number with your cell phone so it won't ring and will go straight to voicemail. Or save the number as a Contact in your smart phone in the Other category, set the ringtone for the Contact to Silent. Uncheck the box to not show the Other category in your Contact list. Now you won't see the contact, your phone won't ring & it won't go straight to voicemail so the person has time to hang up. If you can't select Silent for the contact ringtone web search or search on Zedge for Silent ringtone. Or web search how to make a Silent ringtone.
Not spam. I had a missed call from this number. I called back and found out it's for a correctional center. My search for the number shows a result from Yelp that says the number is for Stafford Creek Correction Center in Aberdeen, WA.
This number is safe and may be used 3 to 4 times weekly.
Have received 39 calls from this number since 01/24/2020- Wish they would quit calling me. I have the number blocked. Nothing else I can do to get them to stop calling.