Cheapest second line available

Does anyone know if you can add a second line to your T-Mobile phone for less than $30?  Is it possible to add a "watch or tablet" line to your second esim for $10 and just use the talk and text? When I try to add a second line to my phone the cheapest plan is a Magna plan but I wouldn't need data so it seems like an expensive option. 

I tried unlocking the phone but I can't because the phone is under a promo and it's not worth paying off the phone as it would cost me $400-$500 in promo money.  I can get another provider for $9 but can't because it's locked.  

There are some posts on being able to pay off your phone and continue to get the promo paid out. I spoke to T-Mobile today and they said that if you pay it off there is nothing left for the promo to pay off so it just ends. 

I'd do the DIGITS line but I have no data service in my area so it's unreliable.  Any help appreciated.