Alerts via push notification can be sent to your customer or to any user in your contractor account. To send SMS alerts, however, you must have an Enthusiast Plan.
NOTE: If your customer has an Enthusiast Plan, they can receive SMS alerts. As a contractor, you won't receive these SMS alerts unless you also have an Enthusiast Plan. However, if your customer has an Enthusiast Plan, you can enter your own number instead of the customer's to receive alerts.