First, there isn’t an exact formula for calculating the needed bandwidth. It depends on the VoiceCodec that you will use.
G.729 is very good VoCodec if you choose it that will cost you about 26.4 kbps (PPP) or 29.6 kbps (Ether) for every channel in TDM. In you case 180*26.4=4.752Mbps
But that is not constant, bandwidth is adjustible. It consist of:(Example is with G.711)
1.Size of the sample (e.g. 80B)
2.IP Header (e.g. 20B)
3.UDP Header (e.g.12)
4.RTP Header (e.g.8B)
Total:120B per packet
The sample rate is 100pack/s (10ms per packet)
so the calculations are: 120*8*100/1000=80 kbps per one call
But if you put two samples in one packet you will save about 16kbps (When you increase the number of samples per packet, you also increase the amount of delay
on each call.DSP resources, which handle each call, must buffer the samples for a longer period of time.)
Second, you must consider that your subscribers not only generate calls, but also receive ones. Consider this.
P.S. If you provide me you e-mail I could send you one document about that problem.