If you are the original owners and originally purchased this equipment brand new, through an authorized Mitel (Silver/Gold/Platinum) dealer (or VAR), then whoever told you this is full of crap.
The problem comes up when customers obtain systems through the secondary (used equipment) market.
If the original dealer sold you a system he obtained from the secondary market then I would suggest calling your attorney.
When reselling a system to someone else you can sell the equipment, but you cannot sell the software nor transfer your license to the buyer.
What's hard for some to understand is that you don't "own" the software; you merely bought a license to use it and I'm pretty sure that license is not transferrable.