Utility for testing Internet connection?

Is there a good utility for testing an Internet connection?

Here is the summary: Two locations, A/B. Both served from same ISP. Some times of day VOIP calls between locations is great. Other times, it is unusable.

Have configured routers for QoS, and I’m experienced with this. The same configuration works fine with other ISP’s.

Thinking if I can setup two notebooks, one at each location, with something that will pound the connection for a day, I can find out (once and for all) whether the service is poor.