If they are are a creditor they more than likely know where you work. And if they don't it's very easy to verify that information even without a pay stub.
Depends on how long the debt has been outstanding.
I don't know of any place that doesn't verify your annual income ( to 'keep their records updated' and also 'notify you of any potential interest rate decreases or credit line increases for your benefit'
)
Payback is generally a certain amount of your debt plus interest, but if you've applied for any hardships where a lender negotiates a monthly fee they want to know ASAP when they can charge you much as much as possible.
You can have a $6000 credit line and they may charge you $50 a month plus interest fees cause they want you to pay it off in 10 years rather than say $500 a month far a year.