Just because they don't seem to be well documented elsewhere, here is a brief synopsis of the parameters that can be passed to DAHDI via the Asterisk Dial application:
Dial(DAHDI/pseudo[/extension])
Dial(DAHDI/<channel>[c|r<cadance>|d][/extension])
Dial(DAHDI/(g|G|r|R)<group(0-63)>[c|r<cadance>|d][/extension])
g - Search forward through channel group and use first available.
G - Search backward through channel group and use first available.
r - Allocate next in round robin search forward through channel group.
R - Allocate next in round robin search backward through channel group.
c - Wait for DTMF digit to confirm answer.
r<cadance#> - Set distintive ring cadance number.
d - Force bearer capability for ISDN/SS7 call to digital.
Channel Groups
dahdi/g1/5551212 dials 5551212 on the first available channel in group
one, searching from lowest to highest.
dahdi/G1/5551212 dials 5551212 on the first available channel in group
one, searching from highest to lowest.
dahdi/r1/5551212 dials 5551212 on the first available channel in group
one, going in round-robin fashion (and remembering where it last left
off), searching from lowest to highest.
dahdi/R1/5551212 dials 5551212 on the first available channel in group
one, searching in round-robin fashion from highest to lowest.
Distinctive Ring
dahdi/4r1 dials channel 4 (presumably an FXS channel), and uses
distinctive ring style one. There are four different distinctive ring
styles, so you could replace "r1" with "r2", "r3", or "r4".
Answer Confirmation
dahdi/1c/5551212 tells Asterisk to dial 5551212 on DAHDI channel 1, and
not consider the call answered until the called party presses #. This
might be useful because of the way analog signaling works. Without this
setting, Asterisk will consider any outbound analog call on an FXO port
to be answered as soon as it has been dialed.
Digital Calls
dahdi/1d/5551212 tells Asterisk to dial 5551212 on DAHDI channel 1, and
that it's a digital call. This is useful when making ISDN calls to set
the bearer capability.