Works both ways. When I moved into my current place, they had a pensioner discount* applied on the plan. Did the right thing and called up their accounts. They said thanks, told me they'd fixed the issue, just pay the discounted amount and next month it'd be normal.
Next month, you'll never guess what happened.. so I called them again, explained again how they had applied it by mistake and asked for its removal. Sure thing, they said. The last phone monkey pressed the wrong button but it's all fixed now.
Next month, go on, take a guess. So I called, told then that this was the final time that I would call them, re explained that I don't want this on my account. Then explained that no, I am not trying to apply for a pensioner discount. Eventually they got it, told me about how the other phone monkeys must have missed the steps. Got a receipt number that time.
So next month, actually let's fast forward a bit. About 5 years later, they sent me a nasty letter demanding documentation proving I'm entitled to the discount in 30 days or else they would cut off my discount. Perfect.
So about a year later, I transferred my phone line to nakeddsl with "not Telstra", still receiving my discount every month.
*This is Telstra's idea of a discount, which means only 1.9x the competition instead of 2x