Possible Clues:

• Sounds like line-out call with numbers to Heather in a manner of speaking
• Sound of a working phone
• Telephone sound
• Old Penny and Gillian Cook find mood indicative of the lineout
• Ringer for old leader putting finger on key pad from the start
• Sounds like it's the last thing you hear before getting through to someone from foreign exchange perhaps
• The buzz before the buzz?
• What's cast outside Kent castle can be heard on the blower
• I got landline working — that means phone may be used
• I got land line relaid - something needed for the phone