Data add on - what gets used first

[VXman]

I have 4gb data per month. If I bought a 10gb add on (£9) which carries on until it runs out which would get used first the 4gb monthly allowance or the 10gb add on.

I was thinking of putting on 10 GB just in case the 4gb runs out but if the 10gb gets used first it's doesn't make sense.

[MSF]

@VXman  The Smarty plan is used first. I have a data add on that has sat there with only a small amount used for some considerable time without expiring, but the data in the monthly plan is always used in preference.