Will a prepaid phone be able to call 911 even if it has no remaining balance and the service has expired?

I'm sending my grandma a Verizon prepaid cellphone for emergency use. It has $10 airtime included and I'm also sending $15 refill card. These amounts will expire if not used, and I'm not sure she would buy more refill cards or keep it active. I've heard that any cellphone will still call 911 even if it has no balance and is not active. Is this true? Could my grandma call 911 in an emergency even if she does not keep up with the service?