Calls diverted to wrong number

[OfficeBirdie]

When my phone is either busy or out of signal, my friends and family have been diverted to a wrong number when they are trying to call me. How can I turn off the divert or correct the number??? 

[Enlli]

You should have a call divert menu in your dialer app.

There is also this way

About Call Diverts
With call diverts you can divert incoming calls to any UK fixed phone or mobile. There are four call diversion options available which you can set up from your mobile:
Not Reachable Divert
To activate ** 62 * phone number # SEND
To cancel ## 62 # SEND
To check status * # 62 # SEND
Divert calls when busy
To activate ** 67 * 901 # SEND
To cancel ## 67 # SEND
To check status * # 67 # SEND
Divert calls when there is no reply
To activate ** 61 * phone number * 11 * no seconds# SEND
To cancel ## 61 # SEND
To check status * # 61 # SEND
Divert all calls
To activate ** 21 * phone number # SEND
To cancel ## 21 # SEND
To check status * # 21 # SEND
To cancel all call diverts
To cancel ## 002 # SEND

[MI5 · Accepted solution]

@OfficeBirdie 

Voicemail guide here Guide: All you need to know about O2 Voicemail 901