Reliable method to find ISI rated Journal
The approach you found - that of looking for the journal in the Master Journal List of Web of Science (WoS) - is the correct one. Pain Medicine is indeed indexed in SCIE and thus in one of WoS' databases.
Note that WoS was originally created by ISI (Institute for Scientific Information), but is now curated by Clarivate Analytics. Thus, the label of an &quot;ISI-rated journal&quot; is not entirely correct, and it is possible that due to this incorrect label, the person with whom you corresponded did not understand what you meant.