Partyline: Can I dial out / into a Partyline system

Yes you can dial out / into an Encore Partyline system 
There are two options a) Use a 3^rd party telephone hybridand then a 2- to 4-wire converter unit like the Clear-Com EF-701M (*make sure the converter is powered bythe PL system).Example: JK Audio telephone hybrid http://www.jkaudio.com/autohybrid.htm

• The operator needs to connect a parallel telephone handset with the 3rd party hybrid at the telephone socket
  
• Making an outgoing call – use the parallel phone to establish the call, then manually press the ON-hook button to keep the line up
  
• Receiving calls – let the hybrid auto answer the call, manually press the ON-hook button to drop the line

b) Use a TEL-14 interface and then a 2- to 4-wire converter unit like the Clear-Com EF-701M (*make sure the converter is powered by the PL system)

• The TEL-14 unit needs to have the “matrix” RJ45 data pinsRX/TX linked together
  
• The operator needs to connect a parallel telephonehandset with the TEL-14 at the telephone socket
  
• Making an outgoing call – use the parallel phone toestablish the call, then manually press the TEL-14 ON-hook button to keep theline up
  
• Receiving calls – let the TEL-14 auto answer the call, manually press the TEL-14 ON-hook button to drop the line

See attached for an overview.