If this is the same concept as what I refer to as tromboning. Hope I can explain this clearly enough.
Lets say you connect two PBXs or switches via Tie trunk, a WAN or LAN to connect two sites for example. A call comes in on one site and is transfered to the second over the tie, 1 trunk channel is in use. When the call is tranfered back to the first switch a second channel is in use and the loop or hairpin is created. Some switches have anti-tromboning so both channels are then cleared
Hairpinning refers to the need for a phone to connect to a PBX then back out to another phone. All analog and digital phones do this.
Some IP Phones can “shuffle” meaning that they can connect directly to each other after the initial call setup.
Hairpinning IP Phone to IP Phone is wasteful and should be avoided. IP Phones cannot shuffle is they do not share a common codec.
A T1, in Europe, is refered to as a PRI trunk line provided by the Central Office and provides a Public Network connection with DDI’s anyone can dial.
A Tie-line is a ‘direct’ connection from one site to another to create a Private Network for voice and/or data and have, in most cases, a ‘transparant’ network.