Here's the book for that Viking unit
https://www.vikingelectronics.com/products/pdf/w1-2000a-ewp.pdf I would try these things.
1: Try hooking your BUTT set directly to the door box and see if you have the same problem.(you don't have to hook it into a trunk port)
2: Try putting a working POTS Line into the trunk port the door box unit is using.
3: Try dialing the direct trunk port on the Telrad system and see if you can talk to someone at the door box unit.
4: Make sure First Digit Timeout Timer is set for unlimited
I don't see the Telrad system dropping the line or having to dial a digit to get this unit to repsond. This might be reaching but maybe the hook switch on the Phone thats ringing is sticking alittle bit. I put these unit in all the time and never had this type of issue. Please post back when you find out the problem.