Hello Robin, - First Question - 1. Pay bills from their mobile phone 2. Pay bills on their mobile phone Both "from" and "on" are correct. For example He pays his bills from his mobile phone. He pays his bills on his mobile phone. They pay their bills from their mobile phones. They pay their bills on their mobile phones. - Second Question - You would only use it in its singular form to refer to a single individual. If you want to be more specific stating that a person uses multiple phones to pay his bills, you could use: He uses multiple phones to pay his bills. He pays his bills from his various mobile devices. He pays his bills using multiple mobile phones. Good luck with your studying !

All prepositions are correct plus some more: “Recently, many people have starting to pay bills from/on/with their mobile phone/phones” — whether or not to use the plural form of “phones” is a philisophical choice here, and “their” is not necessary if you use the plural. If you use “by” don’t use the plural: "Recently, many people have starting to pay bills by mobile phone."