Cellphone contract update - pay for two handset after upgrading

I was given the opportunity to upgrade my contract in Feb 2019 instead of April 2019. I receive great service through the whole process as it was hassle free to upgrade.

This week I have received my tax invoice and my account is much more higher than agreed upon telephonically. When I contact Vodacom, I was informed that I am still liable to pay for the previous handset until May 2019. This was not communicated to me as this would have had a influence in my decision to upgrade my contract earlier. The person who assisted me told me it was always like that. I am with Vodacom since 2006 and never had this type of issue when I upgraded earlier. If this is the case, why do you get the opportunity to upgrade your contract earlier?

I am very upset as I was never informed beforehand that I am still liable to pay for the old device. I work on a strict budget. This is unacceptable and bad customer service.

It would be highly appreciated if this matter can be corrected before the invoiced amount is deducted at the end of the month.