As fornication is haram in Islam, any action leading to fornication is also haram as stated in the following ayah:
“And those who invoke not any other god along with Allah, nor kill such person as Allah has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment.” (Surah al Furqaan, 68)
"Do not come near fornication, for it is indeed lewdness and an evil life-style." (Al-Isra': 32)
A woman’s using her voice to attract men is haram.
Qur’an recommends women to be careful with their voices and speak seriously in order not to take men’s attention and evil desires on themselves. Because, woman voice is attractive and soft, her alluring style of speech such as in a tuneful or too complaisance manner may entice and tempt male. it should not be soft and flirtatious. So a Muslim lady must be careful with speaking in an appropriate way as mentioned in the following ayah;
"O wives of the prophet! You are not like any of the women; if you are wary (of Allah), then (while talking to other men), be not too complaisance of speech lest he in whose heart is a disease should covet; and (if you need to say something, then seriously) say a nice word!” (Surah-al Ahzab, 32)
As a conclusion, the main point what makes it haram is not the relationship's being physical but its including sexual acts leading to fornication. So, a boy and a girl are not allowed to do such actions even through phone. They are only allowed for the married couples.
May Allah swt guide us all to His right path and make us firm on righteous.