A mobile phone, generally speaking (*), is broadcasting the message "
the SIM card I have with the ID XXXX is now available on the network". This ID is called
IMEI IMSI (**) and is unique to a SIM card.
The IMSI is then mapped to a phone number by your provider.
If you break this chain (by going to the network provider and requesting a new SIM card) the cloned card is useless: its IMSI stops to be associated with your phone number (which you may still keep on the new SIM card).
The technical corner: cloning a card into a functioning one is not a simple task (particularly if the card is protected by a PIN). It requires rogue cooperation with the network operator but once such a card is available, it is placed in a SIM-dialer - a device which continuously dials premium (expensive) numbers so that you run out of credit. This is why it is important to have it blocked as soon as possible, especially when you are in a foreign country. Fun fact: this SIM dialer is called in French a "pondeuse", which refers to a hen laying eggs.
That was for the "someone could call on my behalf" part.
You also mention that you are now getting calls from unknown numbers. These calls are not a danger, they are just particularly annoying. You can try to add them to your automatic rejection list and if they persist, then the only way is to change your number (which can be done without changing the SIM card, per above).
The shop may have given your number to scammers, you will never know. Or it may just be a coincidence.
As a side note, beware of calls which just ring once or twice, trying to get you to call back (or if you pick the phone up, there is a generic message like "I cannot hear you, hello? hello? Please call back this is important"). There are usually two categories of such scam calls:
- the ones which show a local number. If you call that number you get a message of the type "you have received a voice mail, please dial to retrieve it"
- the ones which show a premium number with the international prefix for your country. The idea is that when you live for instance in France (prefix +33) you may be less reluctant to call back +33899235467 than an well-advertised 0 899 ... which is more recognizable as an expensive (premium) number.
(*) "generally speaking" means that what follows is mostly correct, there are many simplifications and some inaccuracies but you should get the whole picture.
(**) I initially wrote by mistake "IMEI" instead of IMSI. The IMEI is another identifier, unique unique to the phone itself (not the SIM). Its uniqueness is supposed to help to blacklist a stolen phone but it usually does not work (either the blacklisting is not applied by the carrier, or is reset by the thieves to something else).