Communication speed measures data transmission rate in bps (bits per second). 1Mbps transfers about 1 million bits per second; 1Gbps about 1 billion. "Download" (downstream) and "upload" (upstream) speeds exist, with download typically faster.
Speed directly affects phone service quality. VoIP calls need minimum 100kbps; video calls need 1.5Mbps+. Insufficient speed causes audio dropouts, video stuttering, and lag. 5G theoretically reaches 20Gbps, about 20x faster than 4G.
MVNOs tend to slow during peak hours (noon, 6 PM) due to limited leased bandwidth. For reliable call quality, major carriers or services with dedicated voice lines are safer choices during congestion.
Free speed tests include Speedtest (Ookla) and Fast.com (Netflix). Results vary significantly by time and location, so test multiple times across different hours. See VoIP basics for speed-quality relationships.