Adverts before call goes through

When dialing out I get an unwanted advert. I am very unhappy about this as I use my phone for business. I can not see how this is lawful. I did not give premision for them to do this. <br> I also get competition sms's all the time from MTN even though I rep**** several times with 'STOP'.<br> <br> You charge me for the phone call then you force adverts down my throat! Distgusting!!!!