Over lunch, I did a bit of googling and ran into a new service just started this month, BlueSIP, which offers a 050 phone number. New service is ¥525 to start. Monthly cost is ¥630 per month plus the ¥7.35 universal service fee. Calls are ¥21 per minute to all domestic phones (IP, landline, cellular) and to major countries. There is no discount for calling other BlueSIP numbers.
From what I understand from this IT Media article, it is relying on JCI's (B-mobile) VoIP service. B-mobile does offer IP phone service, but it is so far restricted to the IDEOS. BlueSIP is offering B-mobile U300 SIMs at a 5% discount, so you're looking at a cost of around ¥3500 or so per month for a 300 kbps data connection and an IP phone.
This would not be a good idea to use with a B-mobile Fair SIM because you'd be eating into your limited bandwidth and paying for calls. Docomo flat rate data plans prohibit VoIP, including skype, though I don't know if it would technically work if you tried it. (Docomo blocks UDP ports 2427, 2727, 4661 - 4665, 4672, 4673, 4711, 5060, 5061, 5662, 5773, 5783, 6085, and 6257 with their flat rate data plans for tablets, routers, etc., though I am not sure if they do the same with the unlimited packet plans for phones.)
The service requires downloading an app for making calls. The voice is not routed as traditional voice would be, so you can't use a bluetooth handset. They have confirmed it working on the following handsets:
- Xperia arc (SO-01C)
- SAMSUNG Galaxy S (SC-02B)
- HTC Desire
- HTC HT-03A (above Android 1.6)
- HTC Nexus One
- Huawei IDEOS
- Camangi FM600 Tablet
- iPhone OS 3.2 +