Not Happuy About The Treatment

hello, i was a fido customer for about 3 years i ever get a free phone onece i wanted a phone i asked the customer services to sale me an iphone onece they to told that the phone is 650 but because im a good customer i can have it for $400 but if im leaving the company i have to pay the reast of the money $250 after 5 month i travel and i told them onece i got back i saw they charge me about $600 and they told me i have to pay the whole amount or they will send my information to credite collectors. after few days i receive a call from credit collector asking me to pay that money in rudly way. i want to pay what i owe to fido but not a credit collector how can i do? i want to go to the office or meet somenoe from fido i could not. those credite collector also screwed my credit i dont feel happy because i always pay my bill with fido without any problem give an advice?