A phone can look perfectly connected and still refuse calls. The signal bars only show one part of the connection, and several software switches can silently send callers to voicemail or block them entirely.
Do-not-disturb and focus modes are the most common cause. They can be scheduled or triggered automatically, and they often hide notifications so effectively that the user does not realise the mode is active until they check the status icon.
Call forwarding diverts incoming calls to another number. It can be set intentionally and forgotten, or enabled by a carrier feature. Dialling the status code for your network shows whether forwarding is active.
A loose or failing SIM card can also drop the registration to the network without showing no signal. The phone may still use data over Wi-Fi and appear online while being unable to receive voice calls.