I'll keep my story short as you can read the rest here: http://forum.vodafone.co.uk/t5/Android/S7-Edge-garbled-alien-voice-Nougat/td-p/2556773
Also posted here by another user: https://eu.community.samsung.com/t5/Smartphones-Tablets-Wearables/S7-call-screeching-sound/td-p/1733...
Phone: S7 Edge
Bought: 28th Feb 2017
Bought from: Carphone Warehouse
Problem: Alien-like noise when making or receiving calls. Can happen after 30 seconds or 30 minutes. Happens most calls
When did it start happening: After phone updated to Nougat
Areas affected: NG5 postcode, probably most of NG
Other details: Went back to CPW this week. Woman who works in shop experiencing the same problem after upgrading to Nougat. Chap in shop reckons he's had 50+ returned over the past 3-4 weeks with the same problem. Few phones sent off for repair - all come back as non-faulty. Vodafone checked for problems in area. None found.
Will also raise this as a separate case to get it recognised more and flagged as a separate issue.