I work for a Wireless ISP, all our customers are connected through a wireless network.

We have an elderly customer who I've recently had to replace their Fritz!Box with a newer model as the old one had essentially failed. It's setup as I normally set them up for internet and phone services. They are able to call out and can receive calls fine. 
The Issue: When they try to use their landline for self service phone banking services, they are prompted to enter their customer number however it's not registering that they are entering their customer number and it is again prompting them before they get the chance. I assumed perhaps they were entering it too slowly but that is not the case. This also happens with other self service phone services such as IRD where you're prompted to enter your IRD number.
I know the previous Fritz!Box had a setting enabled that fixed this issue, unfortunately that was years ago and there's no record of what the setting is.

Is this something other ISP's have encountered? Does anyone know what the setting is?

Any help appreciated.