First, it’s wrong to assume everyone has multiple phones. Those with multiple phones are also those who least need to test 911.
But if we can get past that, are you also assuming that the test phone has a phone number? Suppose you call the test center with a friend’s phone (always amusing when a policy imposes having friends, but anyway). The conversation would likely go like this:
me: I will make a test call from a different phone than the one I am calling from.
911 center: Okay, what is the phone number of the phone you are testing (so we know which inbound call to disregard)?
me: The test phone is not subscribed. There is no number.
911 center: Then we cannot do the test.
Or do you imagine the 911 center would say: just call us now and we will ignore whatever call we get?
Of course if a GSM phone has no chip, it also has no number assignment. If the test phone is a VOIP phone, the subscriber may not have a DID. There could be a phony internal number assignment in that case, but the customer does not necessarily know it.
The next problem is /which/ call center do you call when you are on a voip phone served in a different state than where the call is made from?