11 digit contact will not connect - iOS12.4.1

We have a 1300 number that we publish to our customers. They dial through to this service and record information on an IVR service daily. The number is a 1300 PHONEWORDS service - i.e. consists of 11 digits. Recently we have found that customers are unable to even connect - when they dial the service if it is saved as a contact. If however they dial it into the keypad, then connect - no problem. It seems to be an issue with the iOS - not the carrier, as Samsung works fine. This service has been operational for around 6 years without this problem being experienced previously.

Any thoughts??