"Touch to Return to Call" bar non-responsive

I couldn't find a more recent mention of this but I have an iPhone 5S on T-Mobile running 10.3.2 and can't add/merge a call to an existing (incoming) call

I can use the add call button to dial a second number but the green "touch to return to call" bar becomes non-functional and I can't get back to the main 'call screen' to merge the two (indirectly disconnecting the first caller) and have to power down the phone to even end the call with the second number...

Am I doing it wrong???