Customer phone in notification emails

I need to get the customer's phone number in notification emails. We send a lot of shipments from the US to Canada and when shipping across the border UPS requires we also include a phone number. We have a drop shipper who sends everything out for us so the Supplier notification email needs to include a phone number in it.

How can I add this?

Edit the invoice in skins//mail/orders/invoice.tpl

Add in {$order_info.s_phone} where you want.