We recommend at least 100kb in upload and download capacity for each voice call. Lets say you have 10 phones. For all the users to be on the phone at the same time, 10 x 100kb = 1mb upload and download should be available at all times. Often upload speeds are lower than download speeds, so usually focus on the upload speed. You can test your internet speed using https://www.speedtest.net/ You may not always get the full speed that your ISP publishes, but you should see something close. Note that by design, the speedtest itself aims to use your maximum upload and download capacity, and may briefly cause any phone calls to have audio glitches.
The speedtest does also show a ping time. The ping time is the connection time for a round trip of a small packet of data between your location and the speedtest server, sometimes call ping latency. Ideally this should be less than 20-30ms, but as long as this is stable, it can usually be as high as 100ms without a delay that would be noticeable in conversation. Ping stability is as important as the ping time itself. The variation in ping times is called jitter, which can cause glitching in the audio quality. The final parameter of the ping is packet loss. This is where the packets of data containing the conversation get lost somehere in the internet. This can cause severe audio issues, or even dropped calls. How to test your ping time