Ask the caller to check their devices and they need to contact their Carrier.
There is nothing that you can do on your iPhone because the ring tone they get is from the Carrier's server and not from your device
The ringtone that the caller hears when they call your iPhone is indeed determined by the carrier's server.
When someone calls your iPhone, the call signal is sent to the carrier's network. The carrier's server is responsible for managing incoming calls and determining how they should be handled. This includes deciding the ringtone that the caller hears while waiting for you to answer the call.
The specific ringtone played to the caller is typically set by the carrier and can vary depending on factors such as the carrier's policies, the caller's location, or the type of call (e.g., voice call, video call). The carrier's server generates the appropriate ringtone and sends it to the caller's device, allowing them to hear it while waiting for the call to be answered.
It's important to distinguish between the ringtone heard by the caller and the ringtone played on your iPhone. The ringtone on your device is determined by your personal settings, whereas the ringtone heard by the caller is determined by the carrier's server.