Does it cost to use twitter on your cell phone ?

Twitter claims that it does not charge for this service. I have the LG xenon . I got it from bell . I have unlimited texting to only 10 people i can choose from. I tried adding twitters number to my unlimited 10 people list. But it says it is not valid. Will it still cost me ? or is there another number i can use ? (atleast 10 digits i think) PLEASE ANSWER !