Use a mobile phone to make a call, send & receive text and voicemail
3 mobile phones, 1 with voice mail and tincan phones.
Base 1 Use a mobile phone to send an instruction / message via text to the phone at base 2
Base 2 Read the instruction / message sent from base 1, then ring base 3 to tell them instruction / message
Base 3 Answer the phone, remember the message / instruction, then contact base 4 using tincan phones
Base 4 Reject the call from base 3 so it goes to answerphone, then pick up the message
and pass the instruction back to base 1 to check it is accurate.
Move round 1 base,
Repeat until all sixes have had a go on each base