airport time capsule iPhone back up

If you are backing up your iPhone to iTunes on your Mac, then the iPhone backups on your Mac are in turn backed up to the Time Capsule.

So, you could say that the iPhone (and iPad, etc) back up indirectly to the Time Capsule.

The iPhone (and iPad, etc) back up directly to iCloud if you choose that option to back up.

Sorry, but you will not be able to back up your iPhone to a Time Capsule. Instead, you will need to backup to either iCloud or iTunes for that purpose.