Communication speed is the rate at which data moves across a network, measured in bps (bits per second). 1Mbps carries about one million bits per second, and 1Gbps about one billion. Speeds are quoted separately for downstream (download) and upstream (upload) traffic, and the downstream figure is usually the higher of the two.
Speed has a direct bearing on the quality of phone services. As a rough guide, allow around 100kbps for a VoIP call and 1.5Mbps or more for a video call. Raw speed is not the whole story, though. Voice traffic is tiny, so you can have plenty of bandwidth and still hear clipped words or talk over the other party. The usual culprits are three different measurements: how long a packet takes to arrive (latency), how much that arrival time varies (jitter), and how many packets are lost on the way (packet loss). Trouble of this kind rarely goes away by upgrading to a faster plan.
Upstream speed is the part people overlook. Home broadband and wireless links often have a much narrower upstream path, so if the other party sounds fine to you but tells you that your voice or video keeps breaking up, a shortage of upload capacity is the likely cause. Standards such as 5G are described as reaching a theoretical maximum of 20Gbps, but that is a ceiling under ideal conditions. A headline "up to xGbps" figure is a best-effort claim rather than a guarantee, and the speed you actually get shifts with location, time of day, handset, and how busy the base station is.
MVNOs (low-cost SIM providers) tend to slow down during the busiest hours, around noon and 6 PM, because the bandwidth they lease from the major carriers is limited. If call quality matters most to you, a major carrier or a service with dedicated voice lines is the safer choice.
Free tools such as Speedtest (Ookla) and Fast.com (Netflix) will measure your line. Results swing widely by time and place, so test several times across different hours. When you are chasing a call-quality problem, look beyond the download figure: check the latency (ping) value shown alongside it, and compare a wired connection with Wi-Fi. That comparison is usually enough to tell whether the fault lies with the line itself or with your home setup. See VoIP basics for more on how speed relates to call quality.