Sounds like a service tagging error only. Call CS and they should be able to rectify it

I'm using DiGi's BlackBerry Unlimited package and it doesn't affect my unbilled amount.

This is pretty much how the BB Unlimited works:

-unlimited free usage SO LONG you are using it on your BB device (i.e. BlackBerry.net APN)
-unlimited free streaming as well on BB device (use digistrm APN)
-Tethering phone (as a modem) will be charge pay as you use

At RM58 per month for unlimited usage this is really great for me.
Voice calls no commitment.
And since my bills are usually RM200 per month (voice + data) i'll also enjoy the 100% discount on BB Unlimited (which is worth RM58 per month).