I can't select NTT docomo in the field "代表回線以外 (except the main line)" on the online application screen for "Omatome Seikyu Service (combined billing method)."
回答
You can't register NTT docomo lines as sub-lines with "Omatome Seikyu Service (combined billing method)."
If you have multiple docomo lines, please apply for "Ikkatsu Seikyu Service (single billing service)."
Click "here" for applications for Ikkatsu Seikyu Service