This should be such an infrequent occurrence that the cost should be negligible. Surely their $10/month plan has enough margin that this can be covered?
There is likely a cost to the infrastructure necessary to enable calling 911 that scales with the number of users not the number of 911 calls. Where I'm at, there is a 75 cent per month fee added to phone plans to cover the costs of access to 911. If most people are on the free plan, the margin from the few paying customers won't cover it.
