Phone line key to dial out a specific line

I have 3 POTS lines going to a SPA8800, 3 sip trunks in free pbx (sip 1, 2, 3). Outgoing and Incoming routes setup and everything is working the way it should.

The client is using aastra phones, he want the receptionist phone to be able to press line 3 and have it dial out a specific number.

How do i do this?