This may sound a bit of a daft question, but you would be surprised at how many businesses lose out on enquiries because their contact form is too complex. This day in age, collecting data such as email addresses and phone numbers is a lot harder than it used to be, so for this reason many companies try and gather as much personal info as they can and permission to contact the customer later on down the line. This is often the reason for lengthy contact forms that seems to want to know everything. But by doing this, you may actually be losing out on immediate and future custom, so it is vital that you get the balance right.
If your contact form is there as a way of allowing your customers to request a call or email back or to ask a quick question, then keep it brief. Allow them to give you an email address and/or phone number as making both mandatory could mean they don’t contact you at all. If sometimes your customers may need to contact you or make a request where you need further information, then try and create a separate contact form for this purpose.