"Can we sign up for the challenge from anywhere."

yup

Is a phone call the only possibility?

Its the only way I know of

"As my trip will span over 8 or so months, I cannot sign up at the beginning as I will not get enough points within the first 3 months"

Would you get enough to complete the gold challenge early on ( 10,000 miles ) and then be able to sign up for the platinum one after that?

You cazn sign up for it at any time. You could do the 1st bit without any challenge of course.

to see restrictions on mileage accrual , go to http://www.aa.com/apps/AAdvantage/Vi...artnerType=Air
(aa.com , advantage, partners & mileage programs, airlines ) and then just look at the details of each airline

Dave