Chandler wants to give Joey $1,500 to pay bills. After they play Foosball, Joey winds up owing Chandler $500 so Chandler suggests they play a made up game called "Cups". After Joey wins $700, Chandler claims that you have to double it and gives Joey $1,400 and then gives Joey 1 card and claims he wins another $100 bringing his total to $1,500, however Joey is already in the hole owing Chandler $500 so Joey really only won $1,000 or Chandler should have made Joey win $2,000 if he wanted to give him $1,500. HOWEVER, since Joey didn't actually pay Chandler the $500 that he just lost to him at Foosball, and since Joey is pretty gullible and easily distracted it can be assumed that Joey forgot all about the $500. So Chandler "losing" only $1500 to Joey in the fictitious game of "Cups" would still be meet Chandler's goal of $1500.
Since Joey has lived at that apartment for years he should be able to differentiate their phone number to the amount they pay for their phone bill.