Marketing is very important and can make a huge difference to how successful a business is. There are a number of different ways in which you can market your business and some may work better than others which is why it is vital that you are able to track how successful it is to be able to make an informed decision as to whether it is profitable for you to continue with it.
Online advertising can often be easier to track than offline marketing as You can have stats packages installed on you website to see where visits are coming from and what the visitor does when on your site. Also many forms of online marketing such as Google Adwords or Facebook Marketing also have their own software that gives you information about the results you have achieved.
If you are using Google Adwords and struggle to find out if the telephone visitor came from Adwords or the organic results you could have a separate phone number to display on all Google Adwords ads and even set it to show that phone number on the website when they visit your site. This may require some additional programming but can be really useful to help you understand how successful it is.