Updated
Quick fix
Use your phone system’s call-handling rules to ring your desk phone and cell phone together, or forward calls after a set number of rings. Using the business phone app on your cell keeps your personal number private.
Step by step
Decide how calls should reach you
Ring everywhere at once (simultaneous ring), ring your desk first then your cell (sequential), or send everything to your cell (forward all).
Set business hours and after-hours rules
Many systems let you choose different rules for business hours, after hours and holidays.
Use the mobile app if you have one
Answering in your business phone app shows your business number to customers when you call back, not your personal cell number.
Test it
Call your business number from another phone and confirm it rings where you expect, and that unanswered calls go to the right voicemail.
Remember to switch it back
If you set temporary forwarding, set a reminder to turn it off.