So is OP's ISP at fault for not responding within the set time frame?
I don't want to point the finger as I have no first hand knowledge of this particular example, so the short answer is "possibly". The general chain of events in this scenario is that Chorus will send an abandonment notification to the current RSP as follows:
Chorus has received a request to abandon the following service(s):
[UBA] ASID: 1684xxxxxx
[POTS] Phone Number: xxxxxxxx
Address: 1 SAMPLE PL, GEEKVILLE, ZONETOWN 1111
RFS: 28/03/2018 PM
Please respond to [Chorus_internal_email] by 14:00 the next business day if you wish to retain the above service(s).
If we do not receive a response from you the abandonment request will be completed and the above service(s) terminated.
The RSP then has a responsibility to try and make contact with their customer within the required timreframe to confirm if they have vacated or plan to vacate the property. Obviously there are times where an RSP may have trouble getting in touch with their customer if they are not home or the RSP holds other contact information (ie a mobile number) which is not up to date. So there will always be some occasions where contact cannot be made despite best efforts. In those cases, the RSP will generally not respond to Chorus (or at least not within the required timeframe) and the connection will subsequently be terminated - but that does not mean the RSP didn't attempt contact.