Hi there JamesNT,
Thanks for your post and welcome to the community.
Apologies for the problems faced with this, can I ask how is your phone connected?
Also has this only occurred recently?
Let us know,
Just found out today, no change to my system. Tried a different phone, just the same. I have a three phone system and disconnected the base phone (only one with a line connection the other 2 on radio link) to try athe standard phone.