Quote:i need to be done using loops to reduce code
No, you don't. You don't validate "using loops" in WinForms, you respond to events and process things.
What you do is handle events like mobileNumber.TextChanged and in that handler you check if landlineNumber.Text has data. If it doesn't, show the user there is a problem and maybe set the focus to the textbox he needs to fill in.
But ... unlike Console apps, your code shouldn't "force" the user to enter any data or make any decisions in any particular order, unless that data or those choices mean that the kind of data you need is different. And then you separate it so that "common" data and the "choice" are visible together, then when he presses "OK" a different set of data appears for his input based on that choice.
For example, if this was a flight booking system, the first page has name, address, mobile number and a selector for "airport to fly from". He selects the airport and presses OK, so a new page appears with destinations available from that airport.
The idea is not to force the user, but to guide him into making decisions and providing only correct info. That's better, easier and faster for the user, as well as a lot less confusing. Try it! It's not about "reducing code" - code's cheap - but improving the user experience.