The phone and fax numbers are important so clients can communicate with you, ask you for your products or prices, or even sell your products in other regions of your country.
Phone number and Fax numbers should include the characteristics for your country and region, so customers from other countries can also contact you. This is more or less important depending of what your company does, for example, if you are a Travel Agency, you want that clients from other countries call you to hire your services, but if you are a Bakery, it wouldn’t matter if has the country phone characteristics.
If you don’t know which are your country codes, you can consult here