Here are some settings taken from my SPA3102. These settings assume that your BT number is 02072345678, your PBX IP address is 192.168.101, the trunk is called BT-POTS with password abcd1234. Substitute your own values for them. Settings not mentioned should be left at defaults.
For the chan_sip trunk:
Trunk Name: BT-POTS
Outbound CallerID: 02072345678
Outgoing sip Settings:
Trunk Name: BT-POTS
PEER Details:
host=dynamic
username=BT-POTS
secret=abcd1234
type=friend
qualify=yes
context=from-trunk
On the Incoming tab, leave everything blank.
On the PSTN Line tab of the SPA3000:
Line Enable: yes
Proxy: 192.168.1.101:5160
Register Expires: 120
Display Name: BT-POTS
User ID: BT-POTS
Password: abcd1234
Dial Plan 2: (<:02072345678>S0)
VoIP-To-PSTN Gateway Enable: yes
One Stage Dialing: yes
VoIP Caller Auth Method: none
VoIP Caller Default DP: 3
PSTN-To-VoIP Gateway Enable: yes
PSTN Caller Auth Method: none
PSTN Ring Thru Line 1: no
PSTN CID For VoIP CID: yes
PSTN Caller Default DP: 2
Off Hook While Calling VoIP: no
VoIP Answer Delay: 0
PSTN Answer Delay: 4
I may have missed some things; describe what goes wrong for both incoming and outgoing calls.