If you're concerned about discretion, what about a sim card plan? I am not sure what kind of phone you have bought for this purpose, but assuming your personal phone is a smart phone you should be able to just switch in between the sim cards. I know all the major carriers have some version of this, it is what I use for my personal phone (my carrier is 3!) I don't recall if I had to do anything when getting the sim card other than paying the couple euro it cost... and for my monthly bill I just buy a 20Euro topup from the local Spar once a month. It gives me unlimited texts, calls, and data.