Discount Code sent to me by eMail doesn't work

[Hornets_Nest]

I have 2 SIM only contracts with O2 - one of the phones used is an old 3g LG Flip Phone and O2 very kindly sent me a discount code for £100 off any handset to be used by 31st Dec as 3g will end then. However when I attempt to use the code online all I get is a promo code not applicable message.  

Their eMail said:

We’re switching off our 3G network by the end of the year throughout the UK.

You need to update to a 4G compatible device if you want to stay connected and keep using the service. 

We’ve made the swap easier for you with a one-time use code ***********. Just use it at the checkout for £100 off any 4G or 5G phone. The voucher is only available until 31 December.

 
You can also upgrade to a Phone Only plan. You can purchase the phone on its own and keep your current tariff.

[MI5]

@Hornets_Nest 

Looks like you should contact the sales team as they can help with that – message them on Social Media if that works best for you - on Facebook or Instagram https://www.o2.co.uk/abouto2/social-media

Or you can call them on 0800 081 0255

Thanks