I've been switching back and forth between two providers for the last 5 years, and I've always wondered now that I'm back to my first provider if calls/texts to me go directly to A, or go A->B->A->B->A.
(From [0]): "One of the very few countries not to use ACQ/CDB is the UK, where once a number has been ported, calls to that number are still routed via the donor network. This is also known as "indirect routing" and is highly inefficient as it is wasteful of transmission and switching capacity."
And
"Because of its donor dependent nature, indirect routing also means that if the donor network develops a fault or goes out of business, the customers who have ported numbers out of that network will lose incoming calls to their numbers."