Changing services in the middle of the billing cycle

I am so frustrated right now and a very hard lesson learned.  My boyfriend wanted me to go in the store to see about getting a better rate on my phone plan.  I was super happy when I left the store and honestly thought I did get a better deal.  It has been a nightmare ever since.  First of all, anytime you change any service in the middle of a billing cycle, they are going to do everything to make them more money.  Example, 3 months later, I canceled one of my phone lines.  Because it was not at the end of the billing cycle, they charged me for the remainder of the billing cycle even though the phone was not being used.  Then another time (about a month later) I called in wanting to add home internet.  That too was the middle of the billing cycle.  Guess what, I just received my new bill and it was added on the remainder of the month for the internet and also the full next month to my bill.  To my surprise, my bill is outrageous.  I wish I never would have stepped foot in there and changed anything!!!!  Oh and being that my bills have been so much higher that expected, I got behind (for maybe 6 hours) and 3 lines were interrupted so they charged me an extra $20/line adding another $60.  Yeah, never again will I try to go get better rates because it has been a mess and wish I would have stayed with what I had from the beginning.