OK - found they are listed in attributes, they are just inactive.
So I've activated them and set access via scim and all good.
But to me it looks like a bug that admin user by default has 'home telephone number' and you can see that on the 'manage people' 'edit' screen in oxtrust gui even though that attribute is inactive.
it's a good bug - because it pointed me in the direction I needed to find the missing attribute.
I think there are two bugs:
1) inactive attributes display on 'manage people' 'edit' in oxtrust gui
2) the home telephone number attribute should not be inactive by default because it is used by the admin user.
these may or may not be bugs:
1) home phone number, mobile, work telephone number missing from schema docs
2) guide to setting up oxd sms twilio should list the need to make the telephone number attributes active as a step
3) phoneNumber list/array in SCIM / swagger / yaml should be removed (not used)