Is she cheating on me?

I have been with my girl friend for the past 1 year . We have had our good times and now for the past 5 months we have hit rough seas .. Our passionate talks have stopped and we keep fighting for trivial stuff. Our sex lives have come to a stand still almost . My girl friend is going thru depression caused by her folks at home and she says that its also due to her old affair that she feels depressed and does not have an inclination towards sex as the problem is what she says around her and she has to sort the issues first . I have gone the way and tried and help her thru her problems by being supportive to her but off late I came to know that she is been speaking to another person( not her ex) thru nights (long distance call )as long as one hour or so at midnights everyday ..Calls happen during the day too towards end of day and talks extend for more than half hour too. I once got hold of her telephone bill that was delivered and found that she was not telling the truth as When ever I ask her she tells me that its her ex boyfriend who calls her and he is in some issue and she is talking to him and helping him out . But she is not talking to her ex but the new guy at these times and is not willing to be open about it. I feel deeply hurt and I called it off yesterday .. infact even yesterday after I called it off I called her and her line was engaged and requested her to spend 5 minutes of talk with her but she refused saying that she cant hang up the other line .. this was in the night .. when questioned , she asked me what my problem was and that she cant tell me .. My love for her was true and im feeling very lonely now .. do you feel she has cheated on me and do you feel that if a person in so much of depression can ever get involved with some one else who is miles apart and the talks are only on phone to the best of my knowledge .. pls advise should I take back my decision or find some one who is true to me ..