Mobile phone users have been warned to do their homework before making calls or sending texts from now on.
Home and mobile telephone users are now able to switch telecommunications companies and retain their phone number.
The change, introduced yesterday and brought about by a Government-driven number portability initiative, will allow, for example, a Vodafone 021 customer to switch to the Telecom 0274 network and keep their 021 number.
The change is geared at increasing competition among phone companies by removing the inconvenience of having to take a new phone number when switching phone networks.
However, it has prompted a warning from a leading international communications expert who has urged callers to check before making calls to avoid being stung by higher phone charges.
Paul Budde, managing director of independent global telecommunications research and consultancy company Paul Budde Communication, said many special-calling rates applied only when users called an agreed mobile network.
For example, customers on a Telecom Go Prepaid Mates Rates Call Plan pay 49c when calling any Telecom mobile or landline. This cost jumps to $1.39 when calling other networks.
Under the current system the dialling prefix allows callers to easily identify which network they are calling, but from tomorrow callers will need to check.
The same problem could occur with texting. For example, Vodafone's Text 1000 package costs just $9.95 for a thousand texts a month if they're sent to phones on the Vodafone network but that rate jumps to 20c a text if sent to another network.
Mr Budde said it was important that consumers were educated on the changes so surprises did not occur come bill time.
"There should be some sort of education about the changes that are happening, otherwise you can expect problems. If you make such changes there is an obligation on the industry to properly communicate these changes ..."
Telecom and Vodafone said they were doing all they could to ensure the change did not impact on their customers. Both have launched a free-text service allowing callers to check the network a number is on before calling.
Telecom spokeswoman Rebecca Earl said users could text to 300 with the number they want to check.
Ms Earl said customers were also urged to notify their contacts and tell them they had changed networks.
She said the new portability option was not expected to bring about major changes in the telecommunications market.
She said based on overseas experience the company expected only about 10 per cent of the country's users to change networks
* Yesterday also marked the end of the 025 network. About 20,000 customers will be affected by that change.