Check this ShowVDTransferInInteractionDriver toggle from configure-> system-> release → toggles.
When enabled/turned on , you will be observing the icon on the interaction beside the SLA where you will be able to transfer the interactions i.e., Voice and Data Transfer.
Are you doing the call transfer through the interaction driver (warm / blind) or you are transferring the call from the phone channel icon which is there on the right top. I believe you are doing the transfer through the options like warm / blind.
Make sure you can select the following option fo both voice and data transfer.
In the navigation pane of App Studio, click SettingsPega CallDevice capabilities.
Select the Enable integrated voice data transfer check box.