Inbound DISA doesnt work on some phones

I’ve recently upgraded to Asterisk 11.4. When I did the installation everything seemed to work ok, however, I’m now seeing problems with inbound calls to DISA.

When I tested it from mobile, it works fine and also from Landlines but the majority of mobiles have problems. I’ve narrowed the problems down by a series of tests and it appears only the digit 3 doesn’t work. For example, when someone dials into DISA and need to dial extension 3100 the phone doesn’t see the 3. When I do it from another phone it works. It’s very consistent, i.e. a phone that seems to works will work every time and one that doesn’t work will always not work. At first I thought this might be provider or handset but we did a test from two iphone 5s, both on ATT and one works every time and the other doesn’t.

It’s a very strange issue and is driving me nuts.