When talking about habits, compare I drink scotch occasionally. I’ll drink scotch occasionally. The meanings are very similar but the second has a faint suggestion that it’s as a result of something that happened before. It’s a little softer and less declarative. If I were confessing my habit to someone who didn’t approve, I might use the second sentence, because it suggests an unspoken good reason that creates my habit. ‘They’ll (Not ‘they will’) spend hours on the phone’ could be talking about the future, or a habit. You need context to figure it out. If talking about a habit, It’s meaning is similar to ‘they spend’

WILL is a word that can be used in any tense. It can be used for the future, present, and past. WILL is a word with a long history, and did not start as a 'future' word. WILL is often modal, like would/could/should/can etc. That's why it works in the past. Example: Many people WILL have worked hard yesterday.

Sometimes in English we use "will do" when we could almost use "insist on doing" - the idea being that it is unreasonable of the person to do what you are describing. For example, "If they *will* spend hours on the phone every night, they're going to have to expect to pay their share of the phone bill." Here, *will* is pronounced with emphasis, and conveys the idea of the unreasonableness of spending hours on the phone every night, and that the people concerned are doing that *despite* there being good reason not to. Likewise: "My horrible squirt of a brother just told Jake I fancy him!" "Well he *would* just go and do something like that wouldn't he, the little horror!" Compare the question "will you marry me?" - the real meaning is something like "are you willing to marry me?" None of these is really the future tense (or conditional tense), we're just using the same modal verb, will/would. Also compare the phrase "if you must", in an exchange such as the following, between two schoolboys in the playground at lunchtime, one of them already sitting down eating his lunch, and the other wanting to sit next to him: "Is it OK if I sit here to eat?" "If you must..." (the real meaning is - "I would rather you didn't, but I won't stop you" - so it's quite rude).

In the example you give using ‘will’ it is not really being used to refer to an event in the future, but as a form used in English to suggest that something that someone is doing now will continue [in the near future] and could lead to something. It adds an element of significance or expectation.