It can be searched up on BL manually via 6435196, since it is its alternate number.
But unless I am mistaken, this can't be done per API.
If this is the case, API should be made aware of alternate numbers:
* If look up using main number returned nothing, run another search via alternate
numbers.
Depending on your internal architecture and % of items with alternate numbers
it might be an easy job to implement or would require an additional index/cache.