In the ringcentral portal you can forward calls to other non ringcentral devices such as mobile phones by manually typing the number and forwarding to other ringcentral users, but according to support this only works f or the RC Phone app and you can't forward calls to other users using Glip as their softphone.

Please add a feature to forward calls to other users in Glip

I thought manually typing the number would work because it is just an arbitrary phone number, but if you try to receive the call forwarding in Glip it will never happen. You  have to be using RC Phone instead which is odd that this rule applies if its a ringcentral phone number, but not any other phone number.