fritzbox_callmonitor: Wrong call-state when 2nd parallel call is ended.

### The problem

Following situation:
- Call 1 is established. Call-monitor state = 'talking' -> Expected behavior
- Call 1 still established, 2nd call is initiated, Call-monitor state = either 'ringing' or 'dialing' -> Not ideal (would be nice to somehow see that this is call no. 2), but tolerable.
- Call 2 is established. Call-monitor state = 'talking', All attributes change to the 2nd call (Name, duration, device, ...)
- Call 2 is ended, Call 1 is still established. Call-monitor state = 'Idle' -> Should go back to 'talking' together with all other attributes because of Call 1.

### What version of Home Assistant Core has the issue?

core-2023.6.2

### What was the last working version of Home Assistant Core?

_No response_

### What type of installation are you running?

Home Assistant OS

### Integration causing the issue

fritzbox_callmonitor

### Link to integration documentation on our website

https://www.home-assistant.io/integrations/fritzbox_callmonitor

### Diagnostics information

_No response_

### Example YAML snippet

_No response_

### Anything in the logs that might be useful for us?

_No response_

### Additional information

FritzBox 7590AX, FritzOS: 7.56

Start with the fritzbox_callmonitor integration and reproduce the two-call sequence described in the issue. Trace how call state and attributes are updated when the second call ends; done means the integration returns to the first call's talking state and attributes while that call remains established.

Written by the indexing model from the issue text.