By default, SMS messages are sent via GSM. At present T-Mobile charges $2.99 for 300 messages and $6.99 for 1000 messages. Obviously the price of a few drinks at Starbucks which isnt a big hit on the wallet.
However, overage charges for going over your limit do add up. The secret loophole can be accessed with a simple settings tweak on your handset. Settings vary from handset to handset so please review your manual or the manufacturers website for reference.
The key is changing your SMS sending options from the default GSM to GPRS and making sure that GPRS is always ONLINE. Laymen terms: GSM is the technology for making phone calls from your handset. GPRS is the technology that GSM uses for packet-switching protocols allowing internet access.
By sending SMS messages via GPRS, T-Mobile is not able to keep track of the messages sent and received on your handset meaning that messages will not be deducted from your alloted SMS bucket in essence allowing unlimited SMS sending and recieving. There is no guarantee that this will work.
However, I have successfully tested this on both T-Mobile pre-paid and post paid accounts. Results will vary. Not guaranteed to work for all users. Tested with T-Mobile US post/prepaid accts. Please note that the following loophole is not guaranteed to work for all subscribers. Results will vary from subscriber to subscriber.
If by chance you are one of the lucky users that successfully enables this on their phones, congratulations. For everyone else, sorry and please do not call T-Mobile US customer care whining that you cant get this working. Please understand that this loophole is only known to exist for T-Mobile US GSM subscribers. ATT/Cingular customers are also GSM subscribers.
Unfortunately I have not had the opportunity to test this on those networks.