Use the phone number as your SIP destination. Enter the "+" symbol, followed by country code, then the number with any leading zeros omitted. You must first enable 'Tel' for the SIP Username, on the sip.audio website.

For example, a North Amercan number would be something like +122233334444 and a UK number would be +441222333333.

You can also use the Shortcut Editor on the sip.audio website, to save an entry for regularly dialed phone numbers, and then dial the shortcut's name instead of the number.