As long as you keep your account in good standing, it shouldn't matter as long as you are not trying to use a blacklisted device, although I'm sure they can tell you are using a different device if they really want to.
Many people have more than one device and alternate between them based on their mood or needs on a given day .
Posted via the Android Central App on my awesome Note 4
More importantly, why would they care? You are essentially paying for that iPhone5 for 2 yrs. If it is sitting in a drawer collecting dust for those 2 yrs, that's your problem, not theirs. All the 2yr contract is saying is that you will pay at&t for 2yrs & they will NOT subsidize another phone for you during that 2yr period. Even if you got that iPhone on the new Next plans, why would they care if you are using it or not? You are still paying your monthly installments for it.
As prolongExistence said, would they care? They wouldn't. Would they know? Yes, if they looked, but they wouldn't unless the phone was blacklisted (the computer flags that automatically).
There aren't any employees with both access to your phone's IMEI and the free time on their hands to look for anyone to notice. If they have the access, they're hoping to get enough real work done by quitting time so they don't have to work late. (Sales associates and customer service reps can't access what's connecting to which tower, and engineers can but care more about whether it's raining at drive time.)
Yes, they will know. For the most part, they will not care, with one important caveat.
They will care only if you have a non-smartphone plan and you plug a smartphone into it. Even if you block data transmission, they will add a smartphone data plan to your account and charge you for it if you put a smartphone (which they identify based on the IMEI, or unique identifier, for the phone) on their network. If you go back to a "dumb" phone, you can always have that data plan removed.
If you already have a smartphone/data plan you are happy with, you can change phones all you want by simply moving the SIM from phone to phone. If they see it happen more than a few times a month, they may (or may not) question you about it.