I have had several split stays. At one point (maybe about 8-10 years ago), it would just open 180+10, regardless of dates I had booked. Some point, when they changed the website, that +10 was reprogrammed to length of stay. Since then, I have only to book for length of stay. I tested it after the 60 day change, because I wasn't sure if we might extend our last trip and was contemplating a couple of ADRs just in case. Nope...I couldn't access anything beyond my checkout date when I hit that 60 day mark. I know some people have had luck booking beyond their length of stay, but that hasn't been my experience.
As for phone vs. online...always online. You give yourself a 1 hour edge on people waiting on the phones if you do it online. I also think it's significantly faster to do it online. I only look to the phone if there's a problem with the website or a specific ADR, but most times, if there's really a problem...they're also experiencing that problem.