This topic has been discussed in-depth on this forum.
If you have music on your iPhone that is not in your iTunes library when you enable iTM that music will be removed from the device. iTunes Match only scans, matches and uploads music in an iTunes library. It does not do this on an iOS device. You will need to get the music into the iTunes library before you enable iTM.
As to listening to music without a network connection: to do so you will need to pre-download music to the device before you leave the network area.
I understand that in some cases I have to predown load music before I leave a network conection. My questiomn is will I have to predownload all of the music that is already held in my itunes library on my cell phone or ipad. If the answer is yes, than I will simply not turn on itunes matich on my phone.