After installing a Skype for Business environment internally which co-existed with our lync 2010 infrastructure, users lost the ability to forward inbound PSTN calls from external numbers. The fix according to MS (https://support.microsoft.com/en-us/kb/2506083) is to change the Trunk configuration settings:
Open the Skype admin centre and go to the voice routing menu.
Select Trunk Configuration and double click the Trunk that is being used. The setting that is causing the issue is the Refer Support option. This should normally be disabled (in our case its set to third party control). When this is set, after a 5-10 minute wait while the config updates, call forwarding and sim ring started to work again.
This is also possible via the powershell interface:
Set-CsTrunkConfiguration –Identity ‘trunk config name’ –EnableReferSupport $False