Many thanks for that clarification. There is nothing on the plug that extends beyond the faceplate surface so it looks like the release clip has indeed been broken off, misleading me to think that it cannot be unplugged.
You might be able to release it by manipulating it with a thin bladed knife.
If you can't free cable with knife or screwdriver then take of bottom plate the remove middle section shown by yellow arrow and then you should be able to remove cable. Then just put middles cation back and replace front bottom section. You can then buy a new rj11 to rj11 cable for the length you need
Thank you, licquorice, I had marked imjolly’s reply as an acceptable solution and I wasn’t able to repeat this on you solution.