Manually?

[DAHDI/1]
type=trunk
label=Trunk
channel=DAHDI/2
channel=DAHDI/3
channel=DAHDI/4
...

SIP trunks are not limited to a particular number of channels in fop2 configuration.. you just specify the trunk name and it will display all the simultaneous calls you might have over that trunk name

[SIP/myprovider]
type=trunk
label=My provider trunk

Now, many people have their asterisk badly configured, where incoming calls are not matched against a sip peer definition, but they come as anonymous sip calls, in those cases, you won't be seeing the inbound calls over that trunk. The name of the channel is important, you can check the names when you have ongoing trunk calls with the command

asterisk -rx "core show channels"

Look for the output for the channel names, and use that name minus the session id suffix to name the button in fop2 button configuration, for example you might see a call like

SIP/myprovider-383a9d3

The trunk name for the fop2 config would be [SIP/myprovider]

Best regards,

[SIP/CTBC]
type=trunk
label=ctbc
..