How to add an explanatory text message in the checkout form

[KaCo]

Hello everyone,

I know it's probably a trivial concern, but I have not been able to find an answer. My store's checkout form has a phone number field, which I have intentionally left as "optional". However, I want to tell my customers the importance of providing a phone number. This is because many postal services around the world use a phone number as the primary contact method. How can I add an informational message about phone number just under the phone number field? Please see the screenshot for visuals. Any help will be very much appreciated. Thank you.

[Akibhusen]

Which Shopify plan Are you using for your store?

[KaCo]

Hi @Akibhusen , I am using 'Basic Shopify' plan. 

[Akibhusen]

@KaCo ,

JFYI, Shopify Plus (Enterprise) merchants are the only accounts able to make any change to the checkout, all other accounts do not have access to add or remove anything in the checkout pages.

We can add any special notes or text messages on the Order page. The last page of checkout after placement of order.

[KaCo]

Hi @Akibhusen 

Thanks for your reply. Customer needs to be educated right where the phone number field is. Any note in the next page or another place would not serve the purpose. I wanted to write something to the effect of "we recommend you to provide your phone number as it is used by postal services to contact you. We do not use your phone number for any marketing purposes." I am also not in favor of making phone number a mandatory field because it would trigger some customers.    

Now, if you say that it's not possible to add any text in the checkout page as a Basic subscriber, I accept that for the time being. But I have to get this simple change done, without paying a premium to Shopify, to minimize goods returns resulting from customers' not being able to be contacted by their local postal services.