The more I think about it, the more I feel cheated

So I received the invoice for one of my two Vodacom numbers yesterday morning and noticed that there were two handset charges on the invoice. I recently upgraded the handset on this number, so I figured they just forgot to stop charging the old phone and added the new phone. Called customer care and was told that the old phone was not paid off yet, so I still have two more payments left on that phone. Fine, that makes sense... but why was this not the case with my other number upgraded earlier in the year ? There the new handset cost was added and the old handset costs stopped... ditto all of the other 15 or so upgrades which I have had over the last 20 years ? And why did nobody bother to tell me that this was the case ? If I knew that I would have to pay off two phones at the same time, then I would have waited 3 months to upgrade only when the old phone was paid off - who has budget to pay for two phones at the same time ? I understand the explanation, but I feel it was not communicated and something about this is inconsistent with past experience. I can't help feeling cheated... and have another two payments to go on the old handset, apparently.