landline handset and/or cord (Fords Farm RG31)

I freegled all my old landline sets and cords, except one, because I was advised I would no longer need them as I was being transferred to VOIP plus landline. HOWEVER that didn't happen! My provider is still 'working on it'! Now the remaining landline handset is showing a message that there is aproblem with my 'line cord'. My provider says there is no problem but will send an engineer. Need to check if my handset cord is the problem before they turn up and charge me £75 because it is MY equipment not theirs!