At some point (perhaps after an upgrade), one particular iPhone stopped being able to voice call or send SMS to another number using PSTN.
Functionality was OK using data, it affected only dialling via the local phone company. The calling phone had no problem calling any other number, the problem was confined to one specific number.
The receiving device also happens to be an iPhone. When the SIM's were swapped between the iPhones, full functionality returned, i.e. either phone could call the other OK.
How to fix this issue?