Break in contract agreement

This company keeps on phoning to sell deals to a client and after a signed contract they change the amount that was suppose to be the set price which is a ********** act, but if you miss one payment then they have the biggest mouths possible but when they cause problems like these then a person has to understand... tomorrow vodacom will phone me and we will cancel every contract i have with them... regardless of how long of a customer i have been to this company their phone calls drops their service has become obsolete and they overcharge for services