Once more about Lync (Skype for Business) RGS forwarding to PSTN

You need to assign Dialplan and voicepolicy to RGS workflow to let forward to PSTN.

Grant-CSDialPlan –Identity sip:workflow@domain.com -PolicyName "USA_DIAL_PLAN"

Grant-CSVoicePolicy –Identity sip:workflow@domain.com -PolicyName "US-International"