For iPhone 6 and 6 Plus users using M1's 4G data plans, you can now choose to make higher quality phone calls using your 4G LTE network.
Navigate to your Settings App --> Mobile --> Enable 4G --> Voice & Data.
M1's Public Announcement:
https://www.m1.com.sg/AboutM1/NewsReleases/2015/M1%20to%20offer%204G%20Voice%20calls.aspx
