I met the same problem as you. I change my current phone system with VoIP.
So I need an intercom device (one button, mic, speaker in good looking box). I see the prices and I was surpised.
Since I buy my SIP phones for 15 euro each, I was having an Idea to make one of the phones a part and use it as intercom,
but It does not have auto hang up. So when the user as example, press one button (6 as example) which is for dialing the ring group
the phone must pick up automatic and dial, then when one of the ring group picks up and hear who is it on the door and close the phone then the intercome must close the phone also or it will be heard busy noise forever.
So my plan is to make a Pi as a SIP client and hide it somwhere in the building and only mic, button and speaker in to the outside box. I am interested of an autodial part and hangup part.
P.S: You can resolve the problem with the SD card with adding a flash dirve - 4GB as example. Format it as usual when you writes the image. Format the same way the sd card. Then change the partition which must PI boot in the SD card and
lock the card for write from the switch and from the system. This decides a lot of problems to me. Also I add a button for shut it down when it needs to. And also a backup power supply from meanwell with and AMSR1.5-7805-NZ stabiliser.
P.P.S.: I have and project with PI and Arduino with RFID reader which writes the tags with time and date to an external server if there is a connection if not it uses local DB. If you find this as usable - for access control as example write me on firstname.lastname@example.org