Yes, you can create a dedicated plan for this service and set the charging amount to zero. However, it is not advisable to only offer the Live Answering/Receptionist service in a plan because SphereMail charges 20% for every customer subscribed to any of the live answering plans.
For example:
If the customer subscribed to the $99.00/month plan. The customer pays $99, and after deducting the 20% fee, the operator pays us $79.20.