These questions are better asked of the billing department, but I can offer the following surmise:
A bill is sent monthly indicating the amount owing, whether positive or negative, and it is the responsibility of the owner of the debt to contact the debtor to collect that debt. So your sister's bill should have shown she had a credit, and either she could have paid less at the time of the next bill, or requested a refund of the overage.
Your second question is in part answered by the first - Telus' terms of providing service, like most credit situations indicates fees and interest for delayed payment, and no credit or bonus for either early or over- payment. This is one of the reasons I pay my bills by bank transfer at the last possible moment - I keep the money earring interest for me, rather than them.