I don't.

I keep mine in my backpack

When I'm on my Dahon foldie, my phone usually goes in my work bag that goes in my rear basket. On my other two bikes (on the fairly rare instance that I use either of them for commuting) I have top tube bags and that's where the phone goes on them.

Cargo pants side pocket.

Phone numbers come with the sim card (not with the phone) You can use two or more phones using the same sim card, and in international travel, you just switch your sim card for the new country you just arrived in and keep using your good old familiar device you been using at home

Some credit cards include insurance for your device for free if you pay your monthly phone bill through them, Mine covers up to $600 for free up to twice a year with just a $20 premium per incidence, There are options out there for insuring your device,

Quadlock. Without a doubt the best phone holding solution up to 100mph or so. Having said that, the Qudlock folks want to sell you their proprietary wireless charging system. So, the "normal
quadlock case shields your phone from working with ANY wireless charger. Not a feature that I much care about, but it might be important to you.

OtterBox clipped onto my belt. :-)

EDIT: Hahaha... forgot I posted already!! (#94)

I have an Otterbox with a Quad Lock adapter glued to the back. One night, I hit a pothole I didn't see, and I hit it so hard that it made me grunt loudly. I heard something fall, and then I realized it was my phone. The adapter and case had come unglued. Luckily there was no traffic and my phone was undamaged. Arguably, I shouldn't glue an adapter on and should use a genuine Quad Lock case, but I'll keep doing this. I reglued it more carefully this time. I think I'll be OK.

But yeah, Quad Lock. It's well made stuff. Not all plastic is the same, and they use good quality. Jenny Graham used it on her record-breaking around-the-world trip. Towards the end, it broke, and even that is a good testimony. A couple of months ago, they had a sale, and I bought another out-front mount to put on one of my rarely ridden bikes.

I find a small handlebar bag is convenient for things like cell phone, wallet, keys, etc.

I carry it in a holder which sits on my handlebars

I use a fanny pack for my phone, keys, etc, but I have been thinking about a handlebar bag.

I use the Loop micro. This thing here

It's surprisingly sturdy and I've had no issues with it despite some bumpy terrain. I keep my phone in an open-face case. I also live in an area without much rain or cold weather, so keep that in mind. All in all, I really like my system.

Peak Design phone case and mount on my stem has served me well for years. I am on my second stem mount due to the rubber tearing on my original, but PD has lifetime warranty so replacement was free.

Smartphones are generally so waterproof these days that I leave my phone in the stem rain or shine without worry.

Jersey pocket obvs