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Hi @SteveR1260 thanks for posting back and this is a strange one and I can understand how important the service working correctly is for you. I've sent you a Private Message so you can get in touch with the Mod team and we'll be happy to get this looked into further and fixed for you.
@SteveR1260 I've never experienced the problem that you're describing, and as you've already had the handsets replaced, the only other variable that I can think of in Settings is Single/Multi Call. I have mine set to Single and I think it was set to Multi by default.
I have no idea why it would make a difference but it might be worth changing to Single and see if that makes a difference. On the handset, select Menu/Settings/Line Settings/Call Settings and change Call mode to Single call and Save.