Phone calls from Random Numbers

Something strange has been happening ever since I got back from Tanzania. Every so often I get a phone call from a random number in Tanzania (country code 255). The caller ID is always different and annoyingly enough, the calls always come in the middle of the night, anywhere from 1:30am to 7 am. Which would be day time if you were in Tanzania. Sometimes the phone rings every night, other times, my phone is quiet for a week or so. I called my service provider and they told me I can only block specific numbers, not a whole range. Apart from changing my number, they could not recommend other solutions. Sure, I can also turn off my phone at night, but I like to keep it on in case there is an emergency. Is this some kind of elaborate scam? Anyone had this problem before? Any tips on how to deal with it?