*Normally* the notice starts when you request a MAC. Try and see if that is the case with Eclipse, or of it is when the MAC is used by the new provider.
If you migrated a broadband only service and left the phone line with the existing supplier, a MAC *must* have been used to migrate service. There is no way it could have been done without this.
A cease fee should only apply when either you have asked Eclipse to cancel the service outright, i.e no migration, or you end the phone line (PSTN cease). There are instances where migrations to MPF (full) LLU suppliers who take over the phone and broadband can result in a cease but this is often due to mis-reporting or incorrect placement of the migration order by the gaining provider.
Did you move the phone as well to the new supplier, or did that stay the same?