If a NTT docomo line is one of the lines which you want to combine by "Omatome Seikyu Service (combined billing method)," the NTT docomo line should always be the main line.

If there is no NTT docomo line for "Omatome Seikyu Service," please select one of your lines (NTT EAST, NTT WEST, NTT Communications) as the main line.

Regarding the main line of "Omatome Seikyu Service," we ask customers to register NTT docomo lines as the main line because once the phone number changes due to moving or other reasons "Omatome Seikyu Service" will automatically be cancelled.

The phone number of NTT docomo line is not going to change even if you move.

NTT docomo lines are useful in this regard.