It should be "携帯電話を忘れたために れんらくできませんでした". And this means that "Because (I) forgot (bringing) the mobile phone, I could not contact (you)". () parts are omitted in the sentence. In this case, 「ために」 is indicating the reason why the person could not make a contact and it is pretty much the same meaning as 「携帯電話を忘れたから れんらくできませんでした」.