Stopping a form field from being required

I must be doing something really simple wrong, but after trying a few things I can't seem to fix this.

I want to make the phone field optional, but for some reason the below code doesn't do that.

This is my code:

<perch:label for="phone">Phone (optional)</perch:label>
  <perch:input type="number" id="phone" label="Phone" required="false" />
  <perch:error for="phone" type="format">
  <span class="error">Please check your phone number</span>
</perch:error>

And it generates:

<div>
  <label for="form1_phone">Phone (optional)</label>
  <input id="form1_phone" name="phone" type="number" required="required">
</div>

I've double checked and I'm definitely working on the right form (when I edit it and upload, save the contact form and refresh things change), but I can't get rid of that required="required".

As I say, it's probably a really simple fix, but I'm at a loss. Any help much appreciated.