I know this would look like a radical change, but I think an easier approach to tiers would be to sell "units" of 100 each, e.g. a unit of voice would be 100 minutes, a unit of txt would be 100 messages, a unit of data would be 100MB. For each additional unit the customer uses you provide a discount, i.e. the 2nd unit is cheaper than the 1st, the 3rd unit is cheaper than the 2nd, etc.
The problem I have is with the voice tiers, I barely pass the 100 minutes mark but I seem to need to pay for 500 minutes each month, out of which I don't use more than 350 of them. The other solution would be to be a bit more generous on the "S" tier :)
Thanks for listening and for providing a great affordable service for those who don't spend their lives on the phone :)