The following functionality allows you to dynamically require the shipping phone number depending on the shipping country of the customer.

Copy and paste the following code into your store's custom “footer” option. This can be found on the “configuration” page of your store's FoxyCart administration, look for the option labelled “Add custom header and footer code to your templates”. Enable it if it isn't already, and paste the code into the “footer” textarea of that option.

{% if context == "checkout" %}
<script type="text/javascript">
function requirePhone(params) {
    var country = params.address.country;
    var phone_required = FC.json.required_fields.indexOf("billing_phone");
    if (country != "US" && phone_required == -1) {
        FC.json.required_fields.push('billing_phone');
        FC.json.config.template_config.custom_checkout_field_requirements['billing_phone'] = "required";
        FC.Template("checkout").clearOutput();
        FC.checkout.renderCustomerShipping();
    } else if (country == "US" && phone_required > -1) {
        FC.json.required_fields.splice(phone_required, 1);
        FC.json.config.template_config.custom_checkout_field_requirements['billing_phone'] = "optional";
        FC.util.removeError(params.address.prefix + "_phone");
        FC.util.notifyErrors();
        FC.Template("checkout").clearOutput();
        FC.checkout.renderCustomerShipping();
    }
}
 
FC.client.on("customer-address-change", requirePhone);
FC.client.on("ready.done", function() {
    var addresses = [];
    if (FC.json.shipping_address.has_shippable_products) {
        addresses = FC.json.has_multiship ? FC.json.multiship_data : [FC.json.shipping_address];
    }
    if (FC.json.use_alternate_shipping_address || !FC.json.shipping_address.has_shippable_products) {
        addresses.push(FC.json.billing_address);
    }
    for (var i = 0; i < addresses.length; i++) {
        requirePhone({"address": addresses[i]});
    }
});
</script>
{% endif %}