Blame it on number portability: NTT DoCoMo posted its first ever monthly decrease in subscribers for the month of November. DoCoMo lost 17,500 users on a net basis, leaving it with 52.1 million customers. That figure keeps DoCoMo in the top spot as the largest Japanese carrier by subscriber contracts. November marked the first monthly decrease for the carrier since its advent in July of 1992.

Yesterday's reports of rupturing Sanyo battery packs in 1.3 million DoCoMo phones certainly won't help matters for the carrier. Rivals Softbank and KDDI attracted more customers in November following the number portability rule change on October 24. However, it should be duly noted that recent reports suggest Softbank has an odd methodology for counting its pay-as-you-go subscribers.

For more on DoCoMo's subscriber decrease:
- check out this article from the WSJ (sub. req.)