Based on the characteristics of the graph provided, this appears to be an MS Excel generated line chart. The question is the nature of the data used to build the chart and how the data was imported. Do you even have assurance that the data represented is accurate? What time zone do the times represent?
Each data point on this chart represents a day and is connected by a line. Without any other frame of reference, there is no way to understand what these data points are representing. Here are some possibilities:
- Daily maximum/peak
- Daily low
- Daily average
- Daily median
- Another daily measure, such as a 95% value
The other concern is how the data is collected. If collected at five minute intervals (not uncommon), you also may not see high utilization (i.e. one minute with high utilization and four with low utilization won't necessarily look high).
Ultimately, this is all somewhat meaningless as you are trying to solve a specific problem and I don't see how this chart is helpful in that endeavor unless it is providing some sort of relevant, meaningful data. This is something that I don't believe has been established with the information you have provided us.
Keep in mind that your issue with VoIP is just a symptom of the actual problem, not the problem itself. You believe you have found the source of the problem by upgrading your down/up speed, but again without relevant, meaningful data you can't know this for sure. All you know is that the symptom you are aware of seems to have been alleviated. However, this could be somewhat like spraying a numbing antiseptic onto a scrape and saying that because it doesn't hurt anymore the problem (i.e. the scrape) is fixed. The underlying problem is still there, you have simply treated a symptom of that problem.
As Ron has mentioned in the comments, for VoIP you should really be running some proper form of QoS to ensure it has the bandwidth needs it requires. Then, you should start monitoring your network so you can understand what is going on and the data you are getting out of it. This way you will be able to start understanding what is normal and what isn't normal on your network.