Networking kit which has been stolen from an unmanned O2 site means customers in parts of Sussex cannot make calls, use the internet or send and receive emails and texts.
The break-in, at one of O2's East London hubs, is also affecting customers in Kent, and north and east London.
Engineers are working to repair the damage done during the well-organised theft and have said they will endeavour to have customers reconnected to the network by the end of the day.
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