You need AS5300/5350 (note that Cisco has announced the end-of-life for 5300). 128M RAM, E1 or T1 voice ports, adequate number of DSPs. IOS 12.3(6a).

AS5300 comes with 4 or 8 T1/E1s. You will not need more than 4 of these, due to the DSP resource limitation for AS5300. For T1 configuration, the maximum voice resource will be 96, and for E1 120. Gateway must be equipped with the sufficient amount of the flash memory to hold the required IOS (12.3 line). It is 16MB for AS5300 and 32MB for AS5350.