That is ridiculous. I would make some company changes at that point such as no company provided phones or set a reimbursement limit. That's a complete waste especially when he could use Skype. Why can he not use the Skype app on his phone? He can link the caller ID so it shows his number on outbound and the app is not much different. I think on Android you can set the phonebook/dialer to default to use Skype dialer when calling instead of calling using SIM.
He could use a multi SIM phone using an overseas SIM for outbound calls when he travels yet the outbound caller ID will be different consistently.
Alternatively if your office has a VoIP phone service, there is also an app which allows more affordable calling rates when overseas (it's similar to Skype - yet more connected with your office). These are good as it makes for an easy follow me, easy access to voicemail, interoffice chat messaging and web meetings.