This is the number one worry business owners bring up, and it's a fair one. Your phones are how customers reach you, so any disruption feels risky.
Answer: A properly managed switch shouldn't cause downtime at all. The key is sequencing. Your new provider sets up and tests your system in parallel with your existing service, ports your number over once everything is confirmed working, and only then decommissions the old line. If a provider suggests cutting over before testing is complete, that's the actual risk, not the switch itself. Ask any provider you're considering to walk you through their cutover process before you sign anything.