7 second calls

I’m a voip reseller and one of my customers has been sending a strange pattern of calls. They pick a country and, for a few days, send calls to random numbers. There might be one call every few minutes. ASR is low but if they connect, they often last exactly 7 seconds. A few days later, they change country. The customer is vague about what the traffic is. It seems harmless but I’m nervous about what it’s for.

Anyone seen this pattern? Testing of some sort?