I hate it when people make me look at the SPForm code.
I wrote it ages ago before I had much of a grasp of how MODX extras should work.
In the SPForm class code, you'll find this embarrassing bit:
function set_placeholders () {
/* ... */
$ph = array(
'name',
'email',
'subject',
'comments',
);
foreach($ph as $p) {
if (isset($_POST[$p])) {
$this->placeholders['spf-' . $p] = $_POST[$p];
} else {
$this->placeholders['spf-' . $p] = '';
}
/* ... *
}
You can certainly change the caption in the spformTpl chunk and remember that it's a phone number, but to really change it, you'd have to alter that array too.