That's very strange, out of all things being affected by the pandemic, the mobile subscriber base would be right at the bottom. If anything, I'd have expected it to increase as members from rural communities who would otherwise not bother with a mobile phone would decide to take the plunge in order to keep in touch with quarantined loved ones or to manage things they'd have done in person otherwise.
EDIT: It just occurred to me the decrease may be linked to mobile phone farms for clicks/engagement/app installs and how their profitability may have taken a dive.