This may be a little complicated but it should work.
Create a new Partition called something like CC_7751_PT. Then create a Line Group that contains the 7751 line on the call center phones and a DN from each of the Call Center leads phones with a Distribution algorithm of Broadcast. Build a Hunt List that contains the Line Group you just created. Build a Hunt Pilot of 7751 in the CC_7751_PT and put the Hunt List you just created in it.
In your Gateway's Calling Search Space, add the CC_7751_PT ABOVE the Partition that contains the Call Center's 7751 number.
What will happen is an incoming call on the Gateway to 7751 will 'see' the CC_7751_PT first and route the call to the Hunt Pilot you created sending the caller to the Hunt List and finally to the Line Group that will ring all the 7751 phones AND the DN's on the Lead people's phones.
If someone Internally calls 7751, it won't ring the Leads phones. You would have to add the CC_7751_PT to everyone's CSS like you did in the Gateway's CSS.