Javascript Android phone call causes Conditional Call Forwarding Active

[AC Question]

I have created a small website and this includes a button to make a phone call. This button activates Javascript code:

javascript:window.open('tel:'+447736110560);

When I click the button the phone dialler is activated and the phone number "447736110560" is inserted (note that the leading + is lost.

When I then try to connect the call the message "Conditional Call Forwarding Active" is displayed. If I replace the "+" with "0" the same thing happens. However if I manually enter +447736110560 or 07736110560 the call connects correctly.

Any ideas on how to overcome this?

Thanks Paul

Details: Samsung S4 GT-I9505 Android 5.0.1 Chrome browser 41.0.2272.96