UPDATE: As of December 29, 2020, GSIS has changed the process. Here’s the advisory as posted in the GSIS website:
Click on this link for the list of phone numbers and email addresses:
https://www.gsis.gov.ph/downloads/20210120-APIR_Mobile_Numbers.pdf?csrt=6890094582981188344
The above advisory can be found in the GSIS website: https://www.gsis.gov.ph/gsis-urges-pensioners-to-report-online-in-2021/