In order to sign in, go into the Target Phone app and click the button at the bottom. It should log you in immediately. Then you just do everything as normal until it rings. You can accept (green) or reject (red) the call (rejecting doesn't reject the call entirely, it still rings for other TMs who are logged in). Once accepted, you can hold or transfer the call -- use hold to look something up while keeping the guest on the line; use transfer to send the call to another area/TM/TL. The extensions, as far as I know, are 2280 - 2284, but you key them in as 2180 - 2184 when transferring calls.
So if you're in, say, market and someone calls inquiring about home décor, you'd hit transfer, key 2180, and call out that there's a home décor call on 2280. If there's already a call on 2280, it should let you know when you attempt to transfer and you can key another extension instead.
If you're in the department that another TM has transferred a call to, you just dial the number they call out over the walkie: 2280 - 2284.