how much bandwidth does a phone call require and how much is the capacity of a single E1?
An E1 circuit has a raw data rate of 2.048 Mbps, of which 128 kbps are reserved for framing and signaling, leaving 1.920 Mbps for payload.

An uncompressed voice channel requires 64 kbps, but this can be reduced using various forms of compression, such as ADPCM at 32, 24 or 16 kbps. This gives a total capacity of 30, 60, 80 or 120 voice channels, respectively.

Since the bandwidth is allocated for the duration of a call, once N calls are active, it is not possible to add call N+1.

EDIT: However, as you were alluding in your question and subsequent comments, some applications do not use E1 to carry channelized (circuit-switched) voice, but rather use the aggregate payload bandwidth to carry, for example, packetized data. Connecting a cellphone base station to its switching center is one such application.

A modern cellphone might use a codec that needs no more than, say 9600 bps to encode voice. Also, in practice, a person is only speaking about 40% of the time, which means that the average bit rate over the long term for each call is on the order of 4000 bps.

This would suggest that one E1 should be able to carry 480 calls, and given the uncorrelated activity statistics of hundreds of simultaneous cellphone users, it is possible to come very close to this number in practice. Two E1s together should be able to handle 960 calls. Since the voice data comes over the air already compressed and packetized, there's no reason to convert it to any other form for transmission over the E1 circuit. This allows the E1 to carry other kinds of packets as well, bandwidth permitting, such as SMS (texting) and IP (Internet) data.