tmobile line rental

Hi

Just noticed on this months tmobile bill that I've been given half
price line rental.

No idea why. Should be paying £20 a month for 100 anything minutes /
only use around 20 min a month at the moment. On a 18 month contract
and in month 15.

Is this tmobile's way of softening me up to get a new contract in 3
months time?

Thanks.

Rich