This is managed by Apple. You can view your subscription following the steps below:

  1. Open the Settings app.
  2. Tap your name. 
  3. Tap Subscriptions. (If you don't see "Subscriptions," tap "iTunes & App Store" instead. Then tap your Apple ID, tap View Apple ID, sign in, scroll down to Subscriptions, and tap Subscriptions.)
  4. Tap the FaxApp - subscription that you want to manage.
  5. It will display all information about available subscription plans and active subscription plan (if any) for 2ndLine.

Alternatively, tap here to learn more.