iPhone phone keypad not responding for telephone banking

The answering service for one bank that I use requires me to select from various options and also to input my account details, date of birth etc. Although this normally works without any problem on other banks and other phone calls it doesn't work for this particular bank. It's the same on my wife's iPhone. I saw a post that suggested changing from banner on incoming calls go full screen but when I checked my phone it was already on full screen. As this only happens with one bank in Australia (I live in France) I wonder if it is a system problem from the bank's phone system rather than an iPhone problem.