Callwell will use the Active Number shown in the top right-hand corner of your screen. Choose the correct number using the dropdown.
If your number is missing, you can add this via the Profile tab, from the left-hand menu. Don't forget to add any options you have during the call journey to your branch ie "Press 1 for sales and 2 for lettings".
A guide to adding or amending numbers is: here
A handy video on how to solve call issues is here.