Editing error messages and allowing a phone number input in a P&T Contact Form
I found it! It took quite a bit of digging about, but eventually I found the files I needed to edit. In case anyone else is struggling with this same issue, here's what you need to do:

Look in the folder 'craft/app/framework/validators/' for a file called 'CRequiredValidator.php'. Open the file in a text editor and look for the text: 'cannot be blank'.

Part of this line reads: '{attribute} cannot be blank.'

You can change the text 'cannot be blank' to whatever you like and can move '{attribute}' to after the alternative text that you've put in without breaking anything. Save and upload your altered file to the correct location.

Now, look in the folder 'contactform/models/' for a file called 'ContactFormModel.php' (probably the only one there).

By default, it has the following written into it:

    return array(
        'fromName'          =&gt; array(AttributeType::String, 'label' =&gt; 'Your Name'),
        'fromEmail'         =&gt; array(AttributeType::Email,  'required' =&gt; true, 'label' =&gt; 'Your Email'),
        'message'           =&gt; array(AttributeType::String, 'required' =&gt; true, 'label' =&gt; 'Message'),
        'messageFields'     =&gt; array(AttributeType::Mixed),
        'subject'           =&gt; array(AttributeType::String, 'label' =&gt; 'Subject'),
        'attachment'        =&gt; AttributeType::Mixed,
    );


The bits you need to alter to complete the customised error messages are found after 'label' =&gt;

Save and upload your second altered file to the correct location, and check out the new error messages for your form!