I do not fully understand the question,
you say your phones and desktops are in the same subnet. In which case they are also in the same VLAN. From a QoS point of view this might not always be the best thing to do.
Smart port is just a way of automatically configuring a port for voice and data, nomally you would manually create a voice and data vlan (trunking it to the cisco phone).
as you are using Mitel phones ( and I do not know if they have the same intelligence as the cisco phones), you might want to just keep them in the same subnet as your "data devices"
or split the subnet in 2. and let route summarization do the rest (split /24 into 2x/25 for instance)
good luck
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