O2 services have resumed 16 hours after thieves knocked out part of the network by damaging equipment.
Customers in north and east London, east Sussex and Kent were unable to make calls, send or receive texts and emails after services were disrupted.
Mobile phone operator O2 said its network had been restored by Tuesday afternoon after the overnight theft and vandalism of equipment at an O2 office in east London.
A company spokesman declined to say exactly what had been stolen, saying it was a police matter, or how many customers were affected by the outage.
Police are investigating the incident which O2 described as "a well-organised theft".
O2 said it could not offer compensation because "the nature of mobile services means that it is not possible for operators to derive which customers may or may not have used their mobiles in an affected area".
Engineers have "replaced the stolen equipment and repaired the vandalism", O2 added.
In February Vodafone's UK's network was similarly struck by a break-in at one of its technical facilities, affecting millions of customers' voice, data and text services.