People talking on my line when I want to make call

Dear Hello Peter<br> <br> I have 4 contracts with Cell C the last contract was suppose to be R100 for a R100.00 airtime a month<br> after the 2nd month I was told this is an open contract, which I then put a limit on I have in time asked to have my limit increased when I had death in the family. I have had my limit on R250.00 per month and by the 2nd week in the month was it all used up, after I have lodged the complain in January where they have my limit as R400.00 last week was R273.00 still available now the account for January is R1015.00 where I have not asked for a limit as much as this, I am afraid somebody is phoning using the contract peoples account and this is not on.<br> <br> Please assist me I have taken this contract from Cell C when they phoned me.<br> <br> Regards,<br>