How can you block withheld numbers to your mobile phone?

A last week, my partner started getting phone calls to her phone by a caller who withheld their number every time. When she answered they hung up and every night since they are doing it and it is starting to get like harassment. I was wondering if there was any way to block people who call and withhold their number so they are forced to make it visible.How can you block withheld numbers to your mobile phone?
Hi this has been happening to lately i just told the coward that i have been to the police and they have my number so they can moniter my calls (i dont know if they can actually do that) but it seemed to work and now it has stopped hope this has helped but i am a teenager so that might be why it has stopped but i dont no about your situation, hope this hs helpedHow can you block withheld numbers to your mobile phone?
Most providers will change your number totally free of charge and usually within 24 hours if its due to a case of harassment which this clearly is. I'd contact them about it and ask their advice. I don't know of any cases where they 'trace' the call as this is only usually done when its a criminal case and as far as I know they cant block individual callers but I'm not 100% as to whether you can block withheld numbers it may depend on the handset but providers are usually very good with cases like this.
get in touch with your network provider tell them of the calls and they will monitor it. next time the coward calls tell them to stop or else you will contact the police they will then find out the number and get the person responsible and they will be done for harrasment
you could program your mailbox to say that you dont answer withheld numbers, or just answer and say call back with your number displayed and then hang up.
Some phones let you do that. RTFM