Why is call forward failing on my iPhone SE?

We have a duty phone (iPhone SE MMXF3B/A) at work which we use to forward to different duty officer each week. This has been a successful process for years but recently the call forwarding is constantly failing. Every Monday the call forward number is changed and we have noticed that the number will randomly change after a couple of days (normally back to the previous number it has been forwarded to) without anyone touching the phone. We have even tried using the phone with call forwarding turned off and the number still forwarded to the previous number!

I have followed every tip I can find online to try and resolve the issue with no luck, including a full reset of the phone (and the phone is a replacement we ordered when this issue started). I am now aware that several other people at work (who all use the same type of phone) are having the same problem. Does anyone know if this is a wider Apple or iPhone or iPhone SE issue and is there anything new to try to resolve it? The setup we have is incredibly important so we need to get it sorted!

[Re-Titled by Moderator]