Depending on when you purchased your system there may be some charges.
I can only speak from my companies perspective some VARs may handle it differently.
Each 3300 has licensing and upgrade support available from Mitel. If the licensing is up to date and has not expired there are no charges from Mitel for an upgrade.
The Mitel software is available to anyone with access to the mitel Website at no charge.
The company purforming the upgrade may charge for labour and or materials according to whatever agreements are in place.
For My customers with un-expired licensing:
Software - No Charge
Upgrade - Labour Only (2 Hrs for first site, 1 hr more for each additional, more than 10 sites requires more detailed scope of work statements.)
With Expired Licensing
Maintenace Update needs to be =purchased from Mitel
The rest is the same as above.
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Occam's Razor - All things being equal, the simplest solution is the right one.