If you're going somewhere that has bad phone signal or none at all, you may wish to temporarily forward your calls to a landline or another portable handset. Just follow these steps to forward calls on your iPhone:
Step 1 – Go to the Settings screen & select Phone.
Step 2 – Select Call Forwarding and turn it on.
Step 3 - Enter the number where you want incoming calls to ring.
Step 4 - Tap the Call Forwarding button to return to the main Call Forwarding screen.
NOTE: To change the forwarding number, tap the x in the phone number field to get rid of the old number, and then enter the new one.