This is almost inevitable when you try to deploy a variable length dialplan, which is basically what you're doing. There are some ways around it. Here's what I would try:
- Retire any DIDs that end in 805x. In reality, you're only losing 10 numbers, and it will make life easy. ;)
- Prefix your 4 digit translations with an asterisk. Of course, this requires re-training.
As for Unity, I'm pretty sure it's still 'single tennant', i.e. no ability for overlapping dialplans. Each subscriber has to have a unique extension. You can, however, add 'alternate extensions' which I believe will help you.
Another option is to stick with four digits on the phones and configure the voicemail profile so that it adds the appropriate prefix.
I'm sure others will have some comments and suggestions as well.
Lelio
Don't forget to rate the post if you found it helpful.