What's the context? Can you give us more details please?

Both are correct, though in the USA at least, "have to" is much more usual. "Must" is stronger, indicating a necessity, and may sound like an order, depending upon the tone of voice (and the source of the voice, like Jeff says re. your boss). "Must" is used more in British tv shows than it is here in the States, so it presumably is (or was) used more often there.

Both are correct. "Have to" means there is a need to bring along but you don't have to. "must" means a requirement. No matter what, your phone is with you - hence sounds like a command. If the person is your boss, the call is important to you, etc, then "must" is preferable.

i think need to use "have to" cuz "must" sounds rly like command) and "have to" as I know need to use when compel circumstances, like here "if u wont take phone I cant call u"

maybe don't SO, but BECAUSE?