I've tried holster cases for the iPhone in the past and found that they would often pop off my belt if I bent over just so. However, that's not been a problem with the US$49.95 Sena Magnetic Holster for the iPhone 6s/6s Plus from Sena. This means it's the only holster case that I use regularly with my iPhone 6s Plus.
The Magnetic Holster accommodates the smartphone with or without a thin snap-one case attached. However, the iPhone fits in VERY snugly, so it's going to have to be a paper slim snap-on case. I recommend using the Sena holster with an iPhone with only a screen protector applied
The Magnetic Holster offers quick access to all ports. It has a soft velvet lining with a light protective layer, a full top flap with magnetic closure, and a belt clip for positioning along the waist.
As with all Sena products, it's hand-crafted from high quality, full grain, Italian leather. If you need quick access to your iPhone -- imagine whipping it out like a gunfighter in a western movie -- the Magnetic Holster is for you. Just be warned: the Apple smartphone is completely exposed to potential danger when it leaves the safety of the Sena case.