Not even 120 but only 118!
Have a look at
The legal stuff you need to know about Boosters - T-Mobile and it will give you the right figure.
Boosters ? Pay as you go Boosters ? T-Mobile lists it wrong in my opinion!
Not really a great offer either. You have to use your special credit in 20 days or else you lose it and it's only for calls back to the UK or within zone. The 30pounds you spend to get 118 minutes works out at just over 25p/min outgoing!
If it really is just a short trip (up to 1 month) I would instead get a UK o2 PAYG SIM and pay 10 pounds for MyEurope Extra. Outgoing calls then cost just 25p/min (not counting the 10pounds subscription though) but the best thing is you get free incoming calls!
Check out:
My Europe Extra - Calling from overseas - International - O2