Before adding the SIP Truck to FreePBX, please make sure you have created an 
Extension and added it to your 
Phone.
1. Login to your FreePBX Installation and in the top menu go 
Connectivity -> Trunks. Click on the 
Add Trunk button and select 
Add SIP (chan_sip) Trunk.

2. On the 
General tab give the trunk a name.

3. Click on the 
sip Settings tab and add the below details into the 
PEER Details text box. Make sure to 
URL/Host to be what Crazytel provided when setting up your SIP Trunk.
type=friend
qualify=yes
port=5060
insecure=very
host=
sip.crazytel.net.audtmfmode=rfc2833
disallow=all
canreinvite=no
allow=alaw&ulaw

You will now be able to make and receive calls through your Crazytel service.
If you experience any issues, please check out 
FreePBX Quick Tips guides.