Losing business over Apple Store misinformation

I spent 2 days at the Apple store in Somerset Mall to take out a new contract with MTN. My CellC contract was about to expire in the next month or two. The Assistant was VERY aware of this.

The assistant ASSURED me that as soon as my CellC contract expires, I simply ask them to put it on pay as you go. I then take my new MTN (which she activated) sim to MTN and they will port my number.

I have used this number for the last 10 years, all of my professional career. I have thousands of rands worth of branding with that number on and I run a business from that number.

MTN now tells me I can not port a number to already active simcard!!

Losing my number will cost me A TON of business.

I feel that Apple Store should be held liable to cancel my contract and open me a new one so that I can keep my number. Or find another solution with the same result...