Roaming on Softbank Breaks Voice Mail?

I haven’t been getting my voice mail on my iPhone here in Japan. When I got off the plane, it hooked up with Softbank which is the company that sells iPhones locally (using a cute, white dog as a mascot). However, no voice mails come in and when I go to the voice mail tab on the phone, it says it can’t connect.

Then my phone switched briefly to NTT Docomo and behold: two voice mails came in. Could it be that Softbank’s own iPhone support interferes with foreign iPhones’ roaming?