When calling out, the called phone number should be formatted as "1" followed by 10 digits, "011" plus digits for international calls, etc., as is standard US practice. Alternatively, full E.164 number formatting is also supported ("+1" and 10 digits US number, "+44" and UK number, etc.).
Inbound numbers will be presented to your system as "1" plus 10 digits in the Request-URI.
Your originating number (Caller-ID) must be formatted as "1" followed by 10 digits and must be present at least in the username field of the SIP From header. Calls without a valid From calling number will be rejected.
Calls from a toll-free number to a toll-free number (i.e. calls where the called and calling numbers are both toll-free numbers) will be rejected (because they cannot be billed).