OK, Form W8-ECI – stands for effectively connected income (ECI).
So, you'd use the W8-ECI if the person or entity is a recipient of U.S. source income that's (or is deemed to be) effectively connected with the conduct of a trade or business within the United States.
But here, (from the instructions for W8-BEN), ...
Purpose of form. Foreign
persons are subject to U.S.
tax at a 30% rate on income they receive from U.S.
sources that consists of:
• Interest (including certain original issue discount
for, or in expectation of, services
• Substitute payments in a securities lending transaction
• Other fixed or determinable annual or periodical gains,
profits, or income.
If you receive certain types of income, you must
provide Form W-8BEN to:
• Establish that you are not a U.S. person;
• Claim that you are the beneficial owner of the income
for which Form W-8BEN is being provided or a partner in
subject to section 1446; and
• If applicable, claim a reduced rate of, or exemption
from, withholding as a resident of a foreign country with
which the United States has an income tax treaty.
Further, here's the definition of EFFECTIVELY CONNECTED INCOME:
Effectively connected income. Generally, when a
foreign person engages in a trade or business in the
United States, all income from sources in the United
States other than fixed or determinable annual or
periodical (FDAP) income (for example, interest,
dividends, rents, and certain similar amounts) is
considered income effectively connected with a U.S.
trade or business. FDAP income may or may not be
effectively connected with a U.S. trade or business.
Factors to be considered to determine whether FDAP
income and similar amounts from U.S. sources are
effectively connected with a U.S. trade or business
• The income is from assets
used in, or held for use in,
the conduct of that trade or business, or
• The activities of that trade or business were a material
factor in the realization of the income.
I don't know all of the circumstances but it appears to me that the W8-BEN is the one to be used, as what you are describing would not come under the definition of Effectively connected Income.
Hope this helps