One

There's only a phone in our office or there's only one phone in our office?

For an explanation about "a/an" and "one" click on the above link.

One of those men is sick. The sick one has to see the doctor.

For an explanation about using adjectives as nouns click on the above link.

One has to do what one is told.

For an explanation about impersonal pronouns click on the above link.