Generally, you should try to avoid working alone whenever this is possible. However, if you have to work alone, then you need to develop an awareness of the risks and how to minimise them.
Prior to making an appointment with someone you do not know, obtain as much information as possible about the person you are meeting and arrange to meet the person on the Company premises. Always call back the telephone number you have been given to confirm that it is legitimate. If a mobile number is given to you should always ask for an alternative fixed-line number.
If visiting, let your colleagues know where you are going, with whom and what time you are expecting to return. If you think that you are going to run over your original timescales, let your colleagues know.
If you are at all concerned that you are being placed in a dangerous situation through your employment, you must discuss this with your line manager.