To forward calls on an iPhone, one common method is through the iPhone's settings, if supported by your carrier:

1. Open the Settings app.
2. Scroll down and tap on "Phone."
3. Tap on "Call Forwarding."
4. Toggle Call Forwarding on.
5. Tap "Forward To" and enter the phone number where you want your calls forwarded.
6. Tap Back to save.

You will then see a call forwarding icon in the status bar, indicating that call forwarding is active. Alternatively, many carriers support call forwarding via dial codes:

• To forward all calls immediately, dial *72 followed by the forwarding phone number, then press Call.
• To deactivate call forwarding, dial *73 and press Call.
• Some carriers might use different codes (e.g., 21 instead of *72).

For calls forwarded only when busy or unanswered, other codes may be used (like 67 or 61). It is important to be within cellular coverage to set up forwarding through settings. If not available in settings, you can use carrier dial codes or contact your provider for support. This covers the main ways to forward calls on an iPhone comprehensively.