I would calculate the checksum value correctly for each of your strings and enter it in, the 5302's get very upset when it is wrong. It does not take too long to do, and will save a lot of grieff.
Configure your switch so you have a port that is in the voice VLAN with no tagging. Plug your 6 faulty phones into that port let them connect directly to the controller. This will leth them download the firmware again and fix them.
When that's done put them back out on your VLAN'ed enviroment after you have manually fixed the option 125 checksum and they shouldn't break again.
BTW when you reset the phones hold down the * key (maybe it is # i can't remember) when powering them on to clear their cache as they hold on to various settings. You will see the lights stay on for a brief period before the phone starts up as normal, although you probably already know this.
The theory is that having the checksum too long causes the phone to read more bytes than it has pre-allocated in memory. The extra bytes spill over into an area that is used for something else, breaking the phone....eek! The value of the bytes that spill over appears to determine whether the phone breaks or not, adding another factor of confusion! Because the phone can't now hop VLAN's to download its firmware it is completley cut off from the rest of the world.
When this spillover scenario happens you will see the phone get DHCP in the native VLAN and release the address as normal, it should then hop to the voice VLAN and ask for another address. When the phone breaks it hangs after this first stage and never appears to hop VLAN's, it just sits there forever, flashing its lights exactyl as you describe.
The only way to fix it is to let the phone connect directly to the controller and download its firmware again.
Hopefully a new firmware and fixed DHCP helper tool will be out soon.
The "proper" Mitel phones appear to perform better sanity checking on the option 125 information and can happily deal with the incorrect checksum as they appear to use the seperate checksum passed down as part of option 125 instead of the value embedded withing the string.
If you have not yet used it i would strongly recommend spending some time sitting down and using wireshark to sniff how these phones operate, the lack of display on the 5302's does not make fault finding easy. Wireshark lets you see exactly what is going on, and greatly helps with fault finding. Much head scratching can result when you have two phones with the same number trying to register!
