Whenever you send a text message delivery request to your SMS aggregator or carrier, they will perform a carrier lookup to find out which carrier the phone number belongs to. Then the message delivery request is sent to the carrier.
If the phone number is a landline or a bad number, your aggregator/carrier will not send the message but, they will still charge you for the message fees.
Carrier failure rates for text messages sent are 15-18% depending on the quality of the data. There are a host of many reasons as to why messages are not delivered but are still being charged:
• Landlines, VoIP numbers, fax numbers
• Virtual numbers
• Carrier blacklisted numbers
• Suspended or deactivated numbers
• Prepaid cell phones ran out of credits
• Carriers that do not accept text messages
If you scrub the phone numbers prior to sending message requests to your carriers, you can save a ton of money. Here is a great super inexpensive solution: www.SubscriberVerify.com