It depend if the ISP registered the domain in
their name or in the one of the customer.
The register will send notification to all 
addresses listed in the whois (usualy admin, tech 
and billing contact). So naturaly, if the ISP 
registered the domain fully in their own name, 
they are the actual owner of it for the register.
(That's why you should never accept that an ISP
does that)

Cheers,
Yann Stettler