Possibly the trunk you are using to call num1 and num2 does not support two channels (three if it also handled the incoming call that was routed to the ring group).
If that’s not your issue, a log of the failure would be useful. If the outbound legs are using a SIP trunk, also include a SIP trace: At the Asterisk command prompt, type pjsip set logger on
or sip set debug on
according to trunk type. Make a failing call, paste the Asterisk log for the call at pastebin.freepbx.org and post the link here. If you are too new to post links, just post the last eight hex characters of the URL.