Pat, whether you ring the airline or go online, you can book them if the seat plan is loaded, whether that's 90 days, or earlier or later. Both have access at the same time, and the same procedures are followed. Last time I booked mine, I checked the seat plan was loaded online, rang the airline to book them (only because I wanted a question answering) and I spoke to two different people who both told me the plan wasn't loaded. I just booked them online instead.
qatatman, it seems to depend on who you speak to if you ring the airline. Some operators will let you book both if the plans are loaded, others won't, and then there are the one's that just give out totally wrong information. There doesn't seem to be any consistency about it at the moment.