You will need a telephone to run your business so that you can take bookings and so on. If, as is likely, your office is at home and you use your domestic line for business purposes, you should get itemised phone bills so that you know how much you have spent on business calls. You should also allocate a certain percentage of the line rental for business use. You should be able to claim this as business expenditure and set it against your tax bill.
An alternative to using your domestic line would be to install a separate line for business calls, or to use a mobile phone dedicated to business use. A business mobile would enable clients to contact you when you are out working, so it's probably essential to have one and to keep it with you at all times.
The telephone company will bill you every month or quarter for the calls you have made, often plus an amount for the line rental for the next quarter.
It's essential to have internet access and an email address. A website is an excellent way of promoting your business - you could even set it up to accept provisional bookings online. Many different companies offer internet access, email accounts and website hosting. Shop around to get the best deal for the package of services your business needs.