Wal-Mart will offer its own brand name cell phone service starting September 20th.  The underlying carrier is T-Mobile.

The service will be a month to month plan.  Users will need to purchase their phone up front, at an unsubsidized price.  The cost of a plan with unlimited talk and texting, plus up to 100 megabytes of data, will be $45/month.  Additional lines can be added for $25/month.

While the pricing for unlimited talk and texting is low – similar plans from the major carrriers run $70 to $90 a month, with a two year contract – Wal-Mart is being criticized for the pricing of its added data plan.  A gigabyte of data costs $40 on Wal-Mart’s new plan.  For comparison, AT&T charges $25/month for up to 2 gigabytes, T-Mobile charges $30/month for unlimited use.

The new plan, Wal-Mart Family Mobile, is a post paid service.  It does not require a contract.  Users pay at the end of the month for the previous month’s service.  Except for the added data, which is sold in pre-paid buckets that carry over to the next month if the full amount is not used.  The bucket of data can be shared with other phones on the same family plan.

Though users pay for the phone at an unsubsidized price, the phone will still be locked to the Wal-Mart Family Mobile plan.  It will not work on other plans or networks, according to Wal-Mart.

The plan will be rolled out in 2500 Wal-Mart stores next Monday.