Unwanted joining of ongoing call

Hi, I have an issue with W59R handsets connected to a W70B base.

FreePBX 14.0.17

Here is the scenario.

User 1 will pick up a call coming from an external source and start talking with a client. Then, User 2 will pick up his handset and will try to dial out, but instead will join the ongoing call between User 1 and the client. This seems to happen once every week.

Any ideas ?

Edit : User 1 was the one who initiated the call to the client. 10 Seconds later, User 2 joined the call.

Are they sharing an extension? I think you need to provide more detail.