Microsoft Corp says that it is adding modems to Microsoft Network points-of-presence at a rate that should increase capacity by 25% a month. It’s racing to stay ahead of the curve and avoid the problems that America Online Inc is going through. Microsoft won’t say exactly how many modems it has in place at the 400 points of presence that are run for it by UUNet Technologies Inc, but it admits that in some cities like Phoenix, Arizona and Denver, Colorado, the situation is particularly bad. Unlike America Online, which says that the average time its subscribers spend online has doubled since it began its $20 per month unlimited price plan, Microsoft Network executives say the average has increased only marginally in recent months. Microsoft Network pretty much invented the $20 price for online services and has been offering it for longer than America Online, so Microsoft Network doesn’t have to contend with millions of subscribers suddenly freed from paying hourly charges.