We will likely require more information to provide you with a satisfactory response. I will assume you are asking about hairpinning calls through a voice gateway. The general idea is that a call comes into a gateway and is hairpinned back out to the same orignating "network". So, a poor man's sketch of hairpinning a call back to the PSTN:
PSTN ----> GW ----> VoIP
PSTN <--- | |
An example use case would be single number reach or an IP phone that is call forwarded to a mobile phone.
You can also hairpin calls back to the IP network:
PSTN ----> GW ----> VoIP -----> GW2
VoIP <----- | |
An example use case is a variant on Mobile Voice Access where you want to treat a call to a VXML application running on the GW2 but that can't be ran on GW.
Hairpinning can be used to solve very specific problems or it can be an aspect of a particular solution (like SNR/Mobility). Sometimes it can be a result of misconfiguration as well. In this latter case, it isn't good simply because it is unexpected.
In cases where hairpinning is part of the design you need to account for overall impact on utilization for the hardware/software resources involved. So, for the first scenario. You must ensure that your telco circuits have added capacity to deal with the hairpinning. In a T1-PRI scenario (for example) you will consume 2 DS-0 channels for the hairpinned call (I am assuming we aren't talking Q.SIG w/Path Replacement).
I'll stop here. Hopefully this response gets you closer.
Regards,Bill
