Here is a working config for Gradwell with Asterisk. You must make the [context] in iax.conf the same name as your username. Secret is set to your password with them.
iax.conf
[general]
; no register line is needed
[general]
; no register line is needed
[your_gradwell_username]
type=user
secret=your_password
context=from-pstn
extensions.conf
[from-pstn]
exten => 0238AAAAAAA,1,Answer
exten => 0238AAAAAAA,1,Wait(1)
exten => 0238AAAAAAA,2,Dial(SIP/1234,30,t)
Replace the AAAAAAA’s in the exten lines with your DID number. If you have multiple numbers from Gradwell you need to include them all in the from-pstn context
Phone System 