Pri is controlled by the carrier. It will start from 1 and then go up. But if there is a spare lower channel then is in use it should use that.

I remember treating on a 7030 were conveyed from pstn to sip with the same issue.

I think in the end ireprogrammed the dt keys to be in reverse but labeled in the correct order. Ie dt8503 was labelled line 1 on the handset, dt8502 was line 2, etc.

Can't remember if I ever asked or got a response from samsung.
Never tried changing from sequential to distribute.
Was Very rare to have dt keys on pri on our customers systems.