Sounds like another quirk of the privatised telecoms sector.
Used to have fun in the UK. If you sent an SMS to a "land line", a very well spoken young lady used to deliver the message verbally. Great fun sending expletive riddled SMS messages to the local pub, and have this girl deliver them, had the landlady in fits several times

I say the phone company is using VOIP / Internet Phone service providers ...........Its cheaper for the company.....They will not tell you what there up to...l call on my landline if l use the card ,it will send a strange number.......if use voip phone it sends my internet number.......l can send sms though them also.