Every field you add costs you messages. That is not a guess, it is one of the most consistently measured things in web design. Somebody standing in a hardware store deciding whether to enquire will fill in three boxes and will not fill in seven.
For most trades, a name, one way to reach them, and a message is enough. You can ask for the rest in your reply, when they are already talking to you and much more likely to answer.
Test it once from the live site after you publish. It takes a minute and it is the difference between finding out now and finding out when a customer rings to ask why you never got back to them.
Step by step
In the editor, click the contact form.
Turn off any field you do not truly need.
Set where the messages go if it is not you.
Publish, then send yourself a test message from the live site.
If it does not go to plan
Should I ask for a phone number?
Only if you will actually ring them. If you reply by email, asking for a phone number costs you messages for nothing.
What about spam?
We filter the obvious rubbish before it reaches you. If something gets through, tell us what it looked like and we will tighten it.