How does it come that the prefix doesn't show up on the specific place that i have the code assigned for?

In oc-admin i have it enabled as: input[name="SQPhone"]

The field has the following html: <input type="tel" name="SQPhone" id="SQPhone" placeholder="+1-xxx-xxxx-xxx" required="">

It is in a custom made php inquiry form and not from Osclass.

@Hugo
Is script loaded on site? No javascript errors?

I assume yes, since it does work on the registration page

@hugo
it works on registration page, becasue it is added to this page via hooks. But it may not be present on other pages.

Okay, how can I enable it on the single item page for the contact form? (as well on custom created php contact forms)

@Hugo
You must take that JS code function from plugin and put it on place where you need it.

Could you please tell me which specific file?

@Hugo
Checked code.
Go to index.php, find this line:
function sms_js() {

Below, there is condition:
    if(((osc_is_web_user_logged_in() && $location == 'user' || osc_is_register_page() || osc_is_login_page()) && sms_param('verification_account') == 1) || ((osc_is_publish_page() || osc_is_edit_page()) && sms_param('verification_listing') == 1) || ($location == 'sms' && $section == 'item-verify' && sms_param('verification_listing') == 1) || ($location == 'sms' && $section == 'user-verify' && sms_param('verification_account') == 1)) {

change it to:
if(1==1) {
So javascript will show up on every page in footer.