Below are a diffrent types of Codecs that Asterisk can handle

Codec	Speed	Simultaneous Calls over T1 (1.5 Mbps)	Notes
u-law	64 Kbps	24	US Standard
a-law	64 Kbps	24	European standard
G.723.1	5.3/6.3 Kbps	289/243
G.726	16/24/32/40 Kbps	96/64/48/38
G.729	8 Kbps	192	Requires license

GSM	13 Kbps	118
iLBC	15 Kbps	102
LPC-10	2.5 Kbps	614
Speex	2.15 - 44.2 Kbps	714 - 34	Open Codec

With higher compression you can squeeze more calls through the same connection pipe, however, the savings of bandwidth does not come without a cost. The hight the compression the lower the voice quality of the call.