There is only one way to do that - it is weird but I guess it works.
www.skutchelectronics.com/as-801.htm Only works for POTS lines, not PRI.
AND you MUST put EVERY line through these units.
If you have many lines you will have to get multiple units.
How it works is that you run your trunks through this unit (or many units) and connect your On-Hold sources to these units. (or no connection for dead air). Then this unit sends out a tone to the MICS for MOH. You really don't want to hear this tone - and you don't when all is working. When you put a line on hold, this tone goes out. BUT the unit is so fast to detect it and interrupts the line and puts the proper SOURCE onto the line.
Check it out - never used it but I heard it once and it did pretty good.
