May I use the phone in your office?
Once you establish the context of your office, you can use the with anything that can reasonably be expected to be found in and/or associated with the office: the chair (in your office), the desk (in your office), the phone (in your office), the floor (in your office), etc.
I will leave a voice message.
You use a because you are leaving one voice message among many possible voice messages. You don't assume your hearer can identify which voice message you are talking about, other than it is one that you will leave.
On the other hand, you can say
I will leave the voice message.
if you expect that the person you are talking to can identify which voice message you are talking about. Maybe it's one you have talked about before, maybe it's one that the person who you are calling told you to leave. This is similar to
I will forward you the voice message
Do you wanna listen to the voice message?
With the, you assume that the person you are talking to can identify which voice message you are talking about. If you don't assume that, than you'd likely use a in these sentences.