backup lost phone

If the iPhone is lost, it is too late to do a backup. If automatic iCloud backup was enabled, then you should already have the most recent backup. See the 'Automatically back up with iCloud Backup' section in:

How to back up your iPhone, iPad, and iPod touch with iCloud - Apple Support

If iCloud is still synching and someone has not stopped the device, that will occur. Otherwise you are supposed to backup regularly anticipating bad events.