Below is the email from them hope this adds a little clarity
Please ensure that your INVITE conforms to our requirements as no further investigation can begin if the INVITE is invalid.
We MUST see a genuine number in your From: header this needs to be in +E164 International format, it also MUST be capable of being called back to a live answer
Accepted formats are
From: sip:+yournumber@yourIP/Domain
From: “sometext” sip:+yournumber@yourIP/Domain
You MUST provide a P-Asserted-Identity: header this number MUST be from the country of origin of the call & may/may not be the same as number in the From: header, but again this needs to be in +E164 International format, it also MUST be capable of being called back to a live answer
Accepted formats are
P-Asserted-Identity: sip:+yournumber@yourIP/Domain
P-Asserted-Identity: “sometext” sip:+yournumber@yourIP/Domain