WooCommerce Phone format incompatible w/ PayPal

Paypal requires the phone format: xxx-xxx-xxxx, but woocommerce’s “Billing Address” section allows any kind of input [such as (xxx) xxx-xxxx format]. This is causing customers frustration because they have to change their phone number when in the paypal checkout page.

Without messing up WooCommerce, how can I add a note to my customers to use the format xxx-xxx-xxxx? I’d like the phone field to look like:

Phone * (Required Format: xxx-xxx-xxxx)

Thank you!

http://wordpress.org/plugins/woocommerce/