“with all other things being equal” I assume you have not attracted extra traffic through sales campaigns, discounts offered etc so the growth is just more of your standard business traffic.
If so a linear relationship is an acceptable assumption.
15% growth for no effort? excellent–send some my way!!
You may need to consider whether your new line of business is the same as that you have at present. If it was aimed at a specfic age group, the peak time may vary to your existing profile. IF you advertise, when do the adverts take place etc etc
You may also need to look at the expected call handle time for the new business. Your peak hour in terms of calls may vary to your peak in terms of resources if the handle time for the new product varied considerably