All of these commands are executed after the outbound dial peer is matched, and before the digits are sent out of the gateway to the pstn. The command that you decide to use on the dial peer is dependent on the type of behavior you would like the gateway to exhibit.
POTS dial peers, by default, strip any outbound digits that explicitly match their destination pattern. For example, given a destination pattern of 55512.. , the called number sent to the PSTN would contain just the last 2 digits. The first 5 digits would be stripped. To stop this behavior, you would use the command "no digit-strip".
The "forward-digits" command lets you specify the exact number of digits to be forwarded to the PSTN. If the number of digits presented exceeds the number allowed, the rightmost digits are sent. Here are some examples:
I want to dial 123-456-7890
In the below scenario, 456-7890 gets sent to PSTN
dial-peer voice 100 pots
destination-pattern 123456....
forward-digits 7
In the below scenario, 123-456-7890 gets sent out
dial-peer voice 100 pots
destination-pattern 123456....
forward-digits all
Lastly, in this scenario 7890 is sent
dial-peer voice 100 pots
destination-pattern 123456
forward-digits extra
The "prefix" commmand adds digits to the number before sending the call out of the gateway. Example: I want to dial 9-1-123-456-7890
dial-peer voice 100 pots
destination-pattern 91T
prefix 1
port 1/0:15
The original 9 and 1 digits are stripped, as they explicitly matched the destination pattern on the dial peer, and the 1 is prefixed to the remaining number, thus sending 1-123-456-7890 to the PSTN.
I hope that made sense to you.
