It is not normal for your optiPoint 420 HFA phones to be stuck in "initializing" state. These phones are installed and working perfectly worldwide. It does happen from time to time, but not all of the phones! Something is definitely wrong.
You are working with HiPath 4000 V2. That was an era when there were many problems with STMI loadware. Try accessing the STMI via Assistant/Expert Mode or directly via browser and acquire the loadware version info. Post it here. If you need assistance with that process, just ask. What is the part number of the STMI board? Q2316-x? Please post that info. Are you using VLAN Tagging on the STMI & the phones? In SBCSU, is there a subscriber password configured? Is that password entered correctly on the HFA phone(s)? Feel free to post your entire AMO HFAB config for one of your problematic STMI/HG3530 boards. I will be glad to review it.
Testing Recommendation: Remove the LAN connection from one of your HG3530 boards for a TEST. Take one of your "initializing" optiPoint 420 phones and assign that phone a static IP address in the same network as your test STMI/HG3530. Also ensure that the phone is assigned a subscriber number and password, PBX address, Default Gateway, etc which are configured on your test STMI/HG3530. Example: if the STMI is 192.168.0 50/24, make your phone 192.168.0.51/24. Make sure you are using the same netmask in both the STMI (AMO HFAB config) and the phone. The "/24" above represents class C (255.255.255.0). Using a LOCAL power supply for the phone, connect the phone directly to the STMI using either a cross-over LAN cable, or use a simple/non-programable Layer 2 switch (not a Hub!) with a straight-thru cable to the phone, and a straight-thru cable to the STMI LAN port. No other cables should be connected to this Layer 2 switch. Either method you choose, now you have a private LAN between the phone and the STMI. If the phone is configured properly, the phone should register and work 100%. If the "initializing" problem occurs now, then you know that the problem is NOT your network and/or NOT your DHCP server. Regardless of whether it works or not, now try using your POE switch instead of the Local Power Supply and non-programmable Layer 2 switch. No other cables should be connected to your POE switch - only the STMI and the one test phone. Does that make a difference? Post the results!
If this "private LAN" does not work, then the problem could be: (1) Bad STMI (try a different STMI board), (2) AMO HFAB misconfiguration (post the config here), (3) the STMI is configured to use VLAN Tagging but the phone is not, or vice versa (check the config), (4) there is a mismatch between the STMI's loadware and the phone's app software (perhaps the HFA software is TOO new - thus incapable of communicating with the older STMI loadware). In this case you could attempt to downgrade the phone's HFA software, or upgrade the STMI's loadware. You will probably need your vendor for the loadware update. Siemens has a simple process called the "EIP" which uses a Procomm script that can install the most recent/recommended loadware for all the boards in your HiPath 4000 V2 system. If this system has not been kept current with VK patches, loadware could definitely be the problem. Is the service vendor Siemens, or an authorized/trained distributor, or "other"? Good luck.