One of the biggest hesitations businesses have about switching telecom providers is losing their existing phone number, especially one customers have called for years. The good news is that number porting is a standard, well understood process, and a competent provider handles nearly all of it.
Here's roughly how it goes: you sign an authorization with your new provider, they submit the porting request to your old carrier, and your number transfers over, usually without any service interruption if it's planned properly. The typical timeline runs anywhere from a few days to a couple of weeks depending on your current carrier.
The one thing to watch for: don't cancel your old service until the port is confirmed complete. A provider who tells you to cancel first isn't protecting your number, they're protecting themselves.