Unfortunately Sprint is very much behind the other carries in rolling out VoLTE (Voice Over LTE) technology. Sprint still relies on CDMA for voice calls. VoLTE allows your phone to have data service at the same time you're on a phone call. Sprint did have an earlier solution known as "Calling PLUS", but it was only available on certain phones and Sprint decommissioned that service because they were rolling out VoLTE.
As of this writing VoLTE on Sprint is available in many, but not all markets. All we can suggest is to go into your phone settings and check your LTE settings. Make sure anything mentioning VoLTE is turned on. However the bottom line is that if your phone cannot have access to data, no app on your phone can send or receive any information, including the Tailwind app. This means that if Sprint does not have VoLTE service in your area, the auto-open / close features (and all other features) of the Tailwind app (and all other apps) will not work while you're on a call if they rely on sending / receiving data over the internet data connection.