iphone
I have a family member who has dementia. Their iPhone was taken away because they kept dialing 911. I gave the family member an iPod Touch, but they really want an iPhone. I took the SIM card out of the iPhone, but discovered that it is still possible to call 911.
Is there a way to disable the 911 dialing functionality of the iPhone in the United States?
This is, unfortunately, expected behavior. Whenever you activate an iPhone (which you do as part of an OS install) it talks to Apple's servers and does various things like exchange crypto certs for APNS, etc. It also checks the IMEI of the phone to see what carrier Apple sold it to, and validates the SIM is valid for that carrier (if you buy an unlocked iPhone in a country where you can buy either locked or unlocked phones the vendor will actually copy down the IMEI number of a normal locked phone, and then enter in their system where it gets pushed to Apple's servers as an unlocked phone, there is no physical or configuration difference on the phones).
The easiest way to get it activated is (as you surmised) to use an AT&T SIM. You mentioned you have an iPhone 4, you can activate it with that SIM. There are plenty of companies that sell microSIM to SIM adapters, but if you want to do it right now you can just place the micro SIM directly in the iPhones 3GS's SIM try with tape. I would not recommend doing this on a device you intend to keep the SIM in for a an extended period because the SIM is likely to come loose (and jam the SIM tray), but if you are just inserting it, activating it, and removing it you should be fine so long as you are pretty careful with how you apply the tape and not shaking the phone too much.
Sadly (for you), Apple only activates the OS on phones that have a current valid wireless account. The official answer is to keep the valid SIM with the device and take the SIM off the account when selling/donatinge the phone.
If you can't get that original SIM, you still have options. You will need an active SIM card to get things activated. (And likely that phone is locked to AT&T - so it will likely need to be an AT&T SIM) If so, the easiest is buying a $6 SIM from AT&T and they have stores all over the place.
Once activated through iTunes, that SIM will work to activate that device in iTunes even if the account expires or that SIM card is no longer active.
You can also jailbreak and/or unlock the device, but that can be more work if you worry a newer OS might tempt you to re-do the jailbreak or unlock work.
Best Answer
No, you cannot disable it for an US iPhone as US law requires that all cell phones be able to call 911 even if no SIM-card is inserted.
You would need to obtain a foreign iPhone. For example in the EU the emergency services are typically not dialed with 911 but rather 112.