iPhone app instability

I have several users with T-Mobile iPhones with the RingCentral app. The Internet connection and wifi are all stable. However, multiple users have repeated instances where the RC app shows that it is reconnecting, frequently while the user is trying to take or make a call. The iPhones are all on wifi, have the latest version of iOS, and the latest version of the RC app. When a user has this issue, we reboot their iPhone, but it doesn't always help. Any ideas or thoughts? Thanks.