Blocked sim

I bought a phone about 4 years ago apparently it's sim locked...so about 3 months back it just changed to emergency calls only,I recharged with R5 airtime to see what could be the problem but it still doesn't work. Yesterday I went to mtn shop,they told me I should do a sim swap that cost around R140...is it even legal to close someone's sim card without notifying them or to just even close it?