Watchdog Ofcom has ruled that firms which route phone calls across the net must connect to 999 and 112.
Ofcom fears delay could make an emergency worse
Before now net phone firms have not been required to connect to emergency numbers. This ruling applies to those firms which let customers call normal national phone numbers.
Ofcom says the stipulation is needed to ensure that during an emergency people can summon help as quickly as possible.
Phone firms have until September 2008 to comply with the ruling.
Research carried out by Ofcom suggests few people know that net phone services, aka voice over IP, do not connect to 999 or 112, the European Union's universal emergency number for mobiles and fixed lines.
In an Ofcom survey 78% of those using net phone services which cannot connect to the emergency services thought that such a call was already possible on that network or were unaware that there might be a problem.
Ofcom fears that having to find a phone that can call the emergency services during a crisis might make a serious situation much worse.
The ruling does not apply to all net phone firms. Only those that let customers call out to normal national numbers but not receive them or let customers make and take calls from normal phone numbers are affected.
Ofcom started a consultation process on the emergency number issue in July 2007.
Excluded are services that only let their customers call other people on the same voip network or that only let customers receive calls from normal phone numbers.
In March Ofcom issued guidelines for net phone firms which stipulated that they must be upfront about the limitations on their services and tell customers what numbers and services they can and cannot call when they sign up.