In regards to sleazy business practices, I am sure any of these companies would be capable of that. I had a bad experience with AT&T upon leaving, as a result I won't give THAT company another dime again ever.
My situation was when I left AT&T and went to Verizon. I had auto bill pay. I tried to cancel my AT&T phone a good long while before the next bill was due, I am thinking 3 weeks plus. I told them at the end of this billing cycle I want it shut OFF, no more service, we are done, it is over...and they were like OK. Long story short, they charged my CC in the NEXT month's cycle. I was month to month, out of contract. I stayed on hold for like 45 minutes bouncing back and forth between reps, of course they denied ever having the conversation of TURN IT OFF 3 weeks before. I never did get my money back, so I have completely written them off as theives. Just to be safe, I went ahead and cancelled that credit card so they couldn't make any further charges.