The data below represent the call volumes over $20$ quarters from a call center at a major financial institution. Develop a forecasting model for the volume of calls $($in $000$ units$).$

$2016201720182019$

Q$1473544628709$

Q$2513582707725$

Q$3582681773854$

Q$4474557592661$

$($a$)$ Develop the quarterly $($Q$1,$ Q$2,$ Q$3,$ Q$4)$ indexes for the volume of calls