If you are just starting out I'd resist the temptation to just give a price for a whole host of reasons.
Make you're first questions where are they and when they want it done, then give a good reason why THEY should let you assess the work to be done.
Being face to face will up your hit rate by miles, you can price up more accurately and more people see your van outside their neighbours house.
Even though you may feel some pressure and they may be mildly annoyed, by not caving in with a price on the phone, ultimately it will work out better for you, and hopefully for them too.