The company has already cut off access to 140 foreign countries and is eyeing up the remainder - America, Europe and 10 other key countries.
A letter sent last month to service providers - the intermediaries who sell mobile phones to the public - warned that a date for the final stage of the ban would soon be set.
The move is sure to upset loyal customers who like to have the option of making international calls wherever they are, and could lead some to defect to rival networks.
Cellnet estimates only 5,000 to 10,000 of the customers affected by the ban regularly make overseas calls from their mobiles. And those that do will be able to continue if they switch to the company's new digital service.
Gary Bernstein, Cellnet's head of corporate security, said the change is designed to stop crooks from cloning analogue mobile phones and running up massive long-distance charges.
Clones are ordinary mobile phones that have been illegally reprogrammed to fool the network into billing their calls to other, legitimate users. Most of them are used to call Third World destinations, so those were the first areas to be shut out.
But industry observers suggested the move might be part of a scheme to switch high-use customers to Cellnet's digital service, thus freeing up scarce bandwidth on its analogue network.
Faced with a similar cloning problem, Vodafone has offered its customers a new security feature, called Authentication, which was developed jointly by the two established cellular communications firms.
Vodafone customers who do not use Authentication will still be able to call internationally, but will have to pay for any bills run up on illegal clone phones.
Mr Bernstein said the Authentication system would have caused as much disruption as Cellnet's current plan, and would have been a stop-gap measure. Both companies have been ordered by the Government to close their analogue networks by 2005.Reuse content