you are correct there is no caller display included,as you dont use the landline for calls I didn't think this would be a necessity,however I see your point regarding receiving/ignoring callers.
If it were added it would be an additional £1.51 making £12.73 (still cheaper than BT)

Taking Caller ID with BT does not remove the requirement to make the minimum number of calls,in fact all services are chargeable if the minimum is not met,1571,caller ID