The Prophet PBUH said:
"Allaah has decreed for the children of Adam a share in adultery, he will get it by all means, the adultery of eyes is looking; the adultery of tongue is speaking; the soul desires and has a passion; the private parts confirms or falsifies it." (Source: http://sunnah.com/abudawud/12/107)
Noticed that actions towards adultery are not limited to physical contact. In the hadits above, even speaking might lead you to adultery. So in your case, since you said that you and your cousin are limited to phone (i.e. speaking), then the things that make it halal or haram are the things you both said during your conversation. Will Allah approve them or not? If your conversation contains something that Allah dislikes, it might be haram. If you're really-really-really sure that Allah approves, it might be halal.
That being said, I'd still suggest that you refrain from prolonging your relationship with your cousin. Syaithan is clever and persistent. If syaithan managed to convince you to take "small and innocent steps" toward adultery, he might manage to convince you to go all the way. Thus, the best approach is to steer clear as Allah prescribed in Al Quran Surah Al Isra' verse 32:
"And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way" (Source: http://quran.com/17/32)
Allah knows best.