Good day.
Please explain this repeated, consistent occurrence taking place beteen 2 mobile phone users, each on a different mobile phone network in South Africa:

Network1 (MTN) user sends a sms to the Network2 (Vodacom) user.

The Network2 user receives the sms and then attempts to reply and/or send a sms to the Network1 user.

The Network2 user receives a "sent" confirmation for the sms on her mobile phone and is also charged on her itemised bill for the sms.
However, the Network1 user NEVER receives the sms.

To summarise: Network1 user can send send a sms to all. Network2 user can send a sms to all EXCEPT one specific Network1 user.

Advice would be most welcome.