about voice: at the air interface 1 time slots uses 16kbs for voice. for simplification, lets say, 1 trx has 8 time slots, so you need 16kbs x 8 ts. so, 1 trx needs 2x64kbs at the abis interface. now, 1E1 has 32x 64kbs. lets say, 30 x64kbs can be used for voice. it is clear, that 1 E1 can serve 30/2=15TRX if only voice is considered. EGPRS is more difficult and depends on the supplier. Some systems can dynamically switch between voice and EGPRS, other systems require (partly) a fixed allocation of time slots for EGPRS. As a consequence, you may need more than 1 E1 per BTS, if the demand for EGPRS traffic is high.