My customers service department has a team of 6 customers service operators to process customer phone orders, enquires, status of orders delivery schedules and so forth. For the past 3 months, our business sales orders increase which lead to high vol

My customers service department has a team of 6 customers service operators to process customer phone orders, enquires, status of orders delivery schedules and so forth. For the past 3 months, our business sales orders increase which lead to high volume of calls. We have feedbacks and complains from customers regarding the difficult reaching my customes service operators. With the current sales turnover in my organization, I am not ready to increase addition head count. When my customers cannot reach my customer service operator, they shine away to order from my competitors'. What is the best solution to manage all my customers calls satisfaction and not wanting to add additional manpower cost?