Irish mobile phone users are being denied a basic part of the service they pay for because short messages cannot be sent between the State's two mobile networks. However, customer pressure looks set to change this.

The short messaging service (SMS), which allows text messages of up to 160 characters to be sent to mobile phones, works between either Esat Digifone or Eircell and their many roaming partners abroad, but not from one Irish network to the other. The operators admit customers say they want the service and plan to introduce it early next year.

Unlike mobile voice calls, SMS messages can usually only be sent between mobile networks which have roaming agreements. However, it is technically possible to send short messages between the Eircell and Digifone networks without a roaming agreement between the two. A Digifone spokeswoman said most cellular operators worldwide do not exchange SMS messages with rival operators in the same country because "it doesn't make business sense".

But the spokeswoman said Digifone and Eircell are in the process of negotiating rules under which the service can be offered. This should address the fear that SMS messages could be used by either operator to poach customers from the other's network. One of the major forthcoming applications of SMS messages will be advertising. An Eircell spokeswoman said it was ready to offer the service, and that the agreement was about to be signed.

Customers who have tried sending messages between the two networks have noticed in the past that some messages have got through. The companies say this is because they have been testing the service. These tests are complete, but the service will not go live until next January, according to Digifone.

In the meantime, using a technique known as tromboning, determined customers can send SMS messages to phones on the other network. Using the menus on most phones, users can set their SMS message centre number to that of a foreign network. As long as the foreign network allows such a practice (the British Cellnet and Vodafone networks no longer do), messages will then get through, although it may cost more.

Various websites contain lists of numbers of various SMS message centres (for example, www.parsley.demon.co.uk).