If it was phone sex, then yes, that's cheating. If your ex called and you talked about everyday stuff or whatever, then, no, that's a phone conversation. This is a double edged sword, but you might consider telling your gf that she called (you may or may not want to include the phone sex part, that's not my place to tell you what to do. The important thing is you tell her that she called). If she finds out that she called and you didn't tell her, she'd be upset. Then again, she might also be upset if you tell her she called because she might get jealous. However, if it is a serious relationship based on caring and trust, I think she'd appreciate your honesty.