Please note that there are only a limited amount of possible 10-digit phone numbers available in the world (XXX) XXX-XXXX. Given that the area code takes up three digits and is typically determined by region, that usually only leaves 7-digits per phone number in a given region.
Because of this, cell phone carriers will reuse and recycle phone numbers. For example, if I end my contract with verizon and give up my phone number, my number will be transferred to a new verizon customer. The previous owner of your phone number probably forgot to inform some (or all) of his/her contacts that they were changing their number. As a result, you may occasionally have strangers calling you looking for someone else. The previous owner's voicemail box may not have been removed properly. Or, there may be an error with your cell phone carrier's call routing methods. I suggest you contacting your carrier to see if they are aware of issues like this.
rest assured, your phone number is yours and only yours (until you decide to switch numbers in the future). Only you will have access to receiving calls/texts/voicemails/etc at your phone number.