To keep your mobile phone as clean as possible, as you can wash your hands until the cows come home, pop it down at work or wherever, and your hands become soiled again.
The chances of transmission of Covid-19 (or, indeed, of anything else) from a mobile phone that's simply been placed on a surface (rather than when someone else is sharing use of it) are probably so remote as to not be worth bothering about.
Before any of this started I always use anti-bac wet wipes, usually the bigger ones like Dettol surface wipes - which luckily I always had a packet of anyway. Use them on my laptop too - they are great for getting marmalade off the keyboard :-)
I use the same wipes as I use to clean my specs, but that is only to clean the screen of fingerprints. What would you do with keys, loose change etc? Heard that one chain of shops only want customers to use contactless as they don't want to handle money. Tough if you don't use a debit/credit card. Too much overthinking IMO.