There is a slight problem with your friends request. He has 4 cordless phones but I suspect they all connect to one base station. That means they are all the same phone, so to speak.
If they happen to be 2-line phones then that makes it a bit easier but you woudl still have 2 base stations then.
Let's eliminate the cordless phones for the moment, or assume each has its own base station:
If it were me, I would set up an account at
Voip.Ms, test things and then port the number to them. They have UK servers.
I would get two Obi202 because each one has 2 phone ports.
Then I would configure 4 subaccounts (extensions) on
Voip.Ms, each one with its own voicemail and failover, etc. I would assign each to have the same CallerId for outbound calls.
Then I would create a recording, "Thank you for calling the XYZ Company. If you know the extension... enter it, otherwise press 101 for Joe, 102 for Sam, etc"
I would then create the Digital Receptionist (IVR), attach it to the incoming line and fan the results out to the appropriate extension.
If desired, you can have the unanswered calls go to an "operator" extension prior to voicemail, etc.
Each person has the ability to call out or receive calls simultaneously. All calls to customers show the same number is calling. Inbound calls get routed as the customer wishes. Each worker has his own voicemail.
The cost is 2 Obis and a few dollars a month for the phone number to be at
Voip.Ms.
A slightly more expensive proposition (not cordless) is using IP phones that have adaptaer built it, or Obi1xxx type IP phones with multiple line buttons so any worker could pick up someone elses call if necessary.
With a single base station and 4 phones that are essentially extensions off that one phone number, only 1 call in or out can be done at the same time. Usually you can transfer a call to another handset, but no one else would be able to make or receive a call with that type of setup. I assume that is not your goal, thus re-thinking the type of phones in use.