In direct communication, the source explained:

No data is available for external debt, as this instrument is not evident in the sector of monetary financial institutions. 

In response, we have:

1. Archived the series (1996Q1 to 2016Q2) by marking its metadata as "discontinued".
2. Marked its catalog entry.

The series resides in the historical catalog:

• Estonia » Financial » External debt » BPM6

and is:

• DBLPIDSMXUQ.IEST = [DISCONTINUED] External debt: Monetary financial institutions except central bank [S122 + S123] - Short-term - Debt securities, (Mil. EUR, NSA)