BRI ISDN (Basic Rate Interface|) Single Line ISDN - 2 ch GSM Gateway This gateway (ISDN BRI) completely bypasses the local landline telephone company, and the associated charges. The BRI ISDN is easily programmable, so the user can easily configure the unit for direct phone calls through the most economical SIM card and carrier. DTT’s powerful Least Cost Router (LCR) software may be programmed to route calls according to multiple variables, including: prefixes (for all 8 SIMs), time-table, and number of calls, number of free calls on Sim, and more thus allowing the user to take advantage of the lowest tariffs accessible.