There is no retention limit on the activity shown in the Recents section of the mobile app – it includes all the calls that have ever been routed to you or placed by you.
Please note though that, in the Recent section of the mobile app, you are viewing your call history only. To see calls that were not routed to you or placed by you, sign in to the web app to access Analytics, where you can see all calls placed or received across your entire system.