Classification of phone numbers belonging to same client
So, finally i managed to come up with a plausible solution.

First of all, i built a dataset by combining each phone number pair, and calculated some mutual features, like proportion of same location per day, wether it is the same device type or not, the group of base station, and etc.

Having done that, i realised that i only have a positive class, hence, can't employ binary classification straight away. To solve this problem i used what's called a p-u (positive-unlabeled) classification, in which i trained 1000 decision trees with my labeled data points as positive class, and random sample from unlabeled data as negative class. Then, i averaged predictions for each entry. 
This approach has resulted in decent F-score.

Here is a resource where you can learn more about pu classification: pu-classification