Randomly, when you dial a number, before the call connects, they play a freaking advert to you, kinda like when you play a youtube video and it plays an advert before you can see the real video? Like that. It is similar to the ads you hear when you phone their helpdesk while you are "on hold" .
Fortunately you can however cancel the call midway in the advert, and try again, usually the 2nd time the advert isn't there. So it is not everytime you dial, it is probably every n-th call or randomly, not sure what the calculation is.
It is just crazy stuff i tell you., i don't think MTN realizes how much it makes people's blood boil when you are phoning and this happens. It is 10x worse than a call dropping, when i hear the ad i just want to strangle MTN and cancel my contract right there.
EDIT: Irony, Whatsapp voice-calling don't have adverts ..... MTN is killing their own core business with these shenanigans, it's BETTER to use VOIP to make calls than voice calls.