I need help with a phone system that works for our small business

Hello!

This is my first post so please excuse if it gets kind of rambley. I have a small plumbing business, 4 trucks, and recently hired an office manager. I’m having a hard time finding a phone tree system that will work for us. I need the office calls to be transferred to my OM between 8 AM and 4 PM Monday through Friday, then be transferred to me the remaining time. I have a current phone plan and number with Verizon that is linked to one phone. I was looking into Grasshopper, but I’m worried about porting over this phone number as it is on all our trucks and would render this phone from Verizon useless. Please help! Any advice would be appreciated!