When more than one trunk group is allocated to the BSC the
MSC will start to assign channels from the different multiplexer.
We will map them the following way onto MGCP endpoints
Multiplex 0:
0 -> 1
1 -> signalling
2 -> 2
.. -> ..
16 -> signalling (might be 15)
30 -> 30
Multiplex 1:
0 -> 31
2 -> 32
...
Multiplex 3:
..