If your phone (example iPhone) doesn’t have complete call forwarding menu, you can use line code, then, to activate and deactivate Call Forwarding, follow this instructions:
Call Forward Immediate – Send all calls to another number instantly.
enable: **21* (prefix + number) # call
disable: ## 21 # call
verify: * # 21 # call
Call Forward No Reply – Send calls that go unanswered.
enable: **61* (prefix + number) # call
disable: ## 61 # call
verify: * # 61 # call
Call Forward Not Reachable conditions – Send calls elsewhere if your phone is off or not in the service area.
enable: **62* (prefix + number) # call
disable: ## 62 # call
verify: * # 62 # call
Call Forward Busy – Send calls that reach a busy signal to another number.
enable: **67* (prefix + number) # call
disable: ## 67 # call
verify: * # 67 # call
To forward call to the operator Voice Mail use: international prefix + your three digit prefix + 20