I use Stamps.com and it requires a phone number on all international orders. When customers pay by Paypal they almost always enter their phone number so I could get it from the PayPal receipt.
With DC - there is no way to get that information so I have to contact the customer before I can ship the package. With new customers they don't always check their emails and it can delay shipping.
I do ask the customer to provide their phone number to me in the invoice message - but they rarely every read it or don't respond when they do.
I'm waiting on a customer to get back to me now and I see this as being a regular thing.