There are 2 possible ways to forward your lines.
Each phone company (AT&T, Verizon, etc.) provides its customers with an access code to implement call forwarding. Your phone company can provide this code to you along with any instructions. The two most common codes to initiate the call forwarding process are *72 or 72#.
When the proper code is entered, you will be prompted to put in your forwarding number with a prompt or another dial tone. It is always wise to call your office line from your cellphone or some other outside line to verify that the proper connection is established with Dexcomm.
OR
Another possibility is that your in-house phone system has built-in codes for the various call forwarding features. You will have to contact your equipment vendor or refer to your owner’s manual to learn more about this.