Tromboning

"Tromboning" is a wonderful word with at least three possible meanings-

1. Playing the trombone
2. A deviant sexual practice, involving reach-around and directed exhalation^*
3. A piece of telco lingo involving call-routing

A telephone call is said to have been "tromboned" if it's gone through an extra, unnecessary circuit to get to its destination. There is a delightful visual metaphor at play here- imagine a trombone player pulling out the slide, creating a longer and longer route from the mouthpiece to the end.

When calls are routed from one operator's network to another, the originating operator pays the receiving operator for the privilege. These "termination charges" apply when you call you a friend on different mobile phone network, make a call from a mobile to a fixed line, call internationally or from any part of one network to any part of another.

Mobile phone network operators have often taken advantage of light regulation to impose very high termination charges, whereas those operating fixed networks have been legally constrained from doing so. Therefore tromboning provides a sneaky way to avoid high termination charges, by routing calls internationally even when the caller and the callee are in the same country!

It goes like this- You are calling your pal on her mobile. Her operator imposes a very high termination rate to other operators. So your fixed line operator routes the call to its international exchange in a neighboring country. From there, it is routed into the receiving operator's international exchange back in the original country. This is where the termination charge is incurred, and it is much lower than the fixed-to-mobile termination charge. From here the receiving operator routes the call to the callee's phone. In the following diagrams, the "$>" shows where the termination charge is incurred.

Simple Situation

A's Network                            | B's Network
+--------+      +--------------+        +--------------+      +--------+
| caller |----->| A's Exchange |------$>| B's Exchange |----->| callee |
+--------+      +--------------+        +--------------+      +--------+

With Tromboning

                                           
                                                A's Network Country 1 | A's Network Country 2
+--------+      +--------------+      +------------------------------+
| caller |----->| A's Exchange |----->| A's International Exchange 1 |-------------------+
+--------+      +--------------+      +------------------------------+                   |
                                                                                         V
                                                                         +------------------------------+
                                                                         | A's International Exchange 2 |
                                                                         +------------------------------+
                                                                                         |
                                                B's Network Country 1 |                  |
+--------+      +--------------+      +------------------------------+                   |
| callee |<-----| B's Exchange |<-----| B's International Exchange 1 |<$-----------------+
+--------+      +--------------+      +------------------------------+            

Now both participants enjoy a lower quality and more lagged call than they would with a local termination. This technically inefficient situation arises because termination charges are not cost-related or subject to competition. It may occur less often nowadays due to increased regulation, and the appearance of separate international termination rates for fixed and mobile calls.