The best thing you could do is try it, divert your phone to your home while your there and then make a call to your mobile and answer the landline. Stay on the call for at least 15 seconds.
I would the cancel your divert and check your bill on line and see if you have been charged.
The bill is general updated within 4 hours, but can take longer.
If you can see a call on there to your landline and the correct time and it says free, then you know its free. However if you get charged then you know you are going to be charged for it.
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This is on the o2 website regarding diverts:
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Call divert
If you can’t answer your mobile, you can divert your calls to another number of your choice. It won’t cost you a thing to set up a divert, but you will be charged for the diverted calls according to your usual tariff. For example, if you divert to another O2 phone you’ll be charged at your O2 to O2 rate, and so on.
This is also how you set them up too
About Call Diverts
With call diverts you can divert incoming calls to a fixed phone, or to another 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 **62* phone number # SEND
To cancel ##62# SEND
To check status *#62# SEND
Divert calls when there is no reply
To activate **61* phone number *11* no seconds# SEND
To cancel ##61# SEND
o 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
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