Use the phone number as the destination in your dial pad. 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 or ipDTL website, to save an entry for regularly dialed phone numbers. You can then dial the shortcut's name instead of the number.