Mac OS X 10.6 Server is using an older version of Samba which only supports the old Windows NT Domain style system and not Active Directory. The reason Apple stuck with such an old version of Samba is that the new versions of Samba now use GPL3 licensing which Apple found to be in conflict with their operation as a business.
For Mac OS X 10.7 Lion Apple have written their own replacement for Samba. At this point we (or at least I) don't know for certain how well this will act with Windows 7 clients.
However as I mentioned some tweaks have been found to help at least partially in supporting Windows 7 clients with Mac OS X 10.6 Server. See this thread https://discussions.apple.com/thread/2200942 I suggest you carefully read it all.
My suspicion is that this will still not let you use Open Directory to manage Windows clients in the same way that a Windows server (via Group Policies) could. It should however hopefully let you use Roaming Profiles.
What do you mean by external network storage server? This sounds like a NAS box. The ones I have seen e.g. a ReadyNAS cannot link to Open Directory and as such you would have to separately define a list of users in the NAS box which makes managing the permissions much harder and less powerful. It would be possible to use a SAN system with Mac OS X Server. What make and model is it? Some can connect to Active Directory but as I mentioned I have not seen any which connect to Open Directory, this is not because it could not be done, but merely because they have not bothered to add that ability.
A VoIP desk phone needs to know how to contact the VoIP controller. This information is normally provided by DHCP option codes. The DHCP server will not be the VoIP controller it will merely advertise how to reach the real VoIP controller, e.g. a Cisco CallManager.
A HFS+ formatted disk can be read by Windows if you install a suitable utility. However as I previously mentioned, the clients would only be accessing the files via a file server and they would never be directly talking to the hard disk and never need to know what format the actual disk is. For example nearly all NAS boxes use a Linux operating system and have the disks formatted in either Ext3 or Ext4 format which are typically only used on Linux systems. A Mac or Windows client would not know how to handle Ext3 or Ext4 format disks but it does not need to because it will speak either AFP or SMB to the server which itself handles the access to the disks.
All the client does is in your case use SMB commands to ask the server to copy a file, or delete a file, or rename a file, etc.
If you were using the Mac OS X Server as the file server, you would use its utilities i.e. Server Admin to share a particular folder, and to assign ACLs to that folder and to folders and files within it.
A HFS+ formatted disk locally connected to a Mac could be used as a Time Machine backup disk. Time Machine is Mac only so will not help your Windows clients. It is also possible to share a disk to Mac clients over a network in a way that will also support Time Machine backups but again this is for only Mac clients. While NAS boxes do not support linking to Open Directory many do support acting as a network Time Machine backup drive including the ReadyNAS range made by NetGear.
You might be able to have a Mac OS X Server connect to a NAS box via NFS and then 'reshare' it via SMB to the Windows clients. This would then allow the Mac server to set and control the ACLs.The Windows clients would be talking to the Mac server and not to directly to the NAS. If one was starting from scratch a simpler approach would be not to use a NAS (since it does not support Open Directory) but to get an external drive enclosure with built-in RAID which connects to the Mac server either via FireWire 800 or eSATA or FibreChannel. For example see this product range http://www.promise.com/storage/raid_category.aspx?region=en-global&m=192&rsn=40& statistic=Mac. These type of products are not called NAS (Network Attached Storage) nor are they called SAN (Sorage Area Network), but instead are DAS - Direct Attached Storage. That is directly attached to the server (not to the clients). They can be equally attached to a Windows box and formatted as NTFS but only attached to a single host (server).