I just got this all working (you can read the trials and tribulations of dealing with Bell in other posts), and in e-mails TSI sends me they mention you can monitor your usage at myteksavvy.com and when I do it always says zero.
IIRC, the agent said monitoring is not until April 1, and the e-mails seem to be contrary to that, though the link then says 0 so far.
I am on a part first month and I doubt I can get close to 300GB anyway but at the same time I'd rather not exceed it unless it's not something to worry about for another week .
