I'm phoning because we'll have to postpone our appointment from Monday until Tuesday._Postpone_ means to _put later_, from the Latin _postponere_: _post_ means _after_ and _ponere_ means _to place_ or _to put_. After _because_ you could use different verb forms, for example _we have to postpone_ or _we are going to have to postpone_, though they have subtly different meanings. Note also that a full sentence structure is used here after because, in contrast to the phrase _because of_, which would be followed by a noun or noun phrase, for example: _because of a change in our schedule_.
