To begin with, best to check that your phones can dial this number. If they can then you already have a rule that allows calls there. If not, then callback should also fail.
Then check your logs in Verbose mode to see why the call fails. Perhaps there is something that causes this that you are not aware of, so best to find out before making any changes
It may matter if you don't have e164 properly set-up to convert the + to a usable prefix, for your outbound rules. What is the error in the Activity Log when a call fails?
You can have the system use a specific trunk group, if you wish, based on the leading digits.
I currently don't have any outbound rules that say anything if there is a "+" in the front. Do you think it's worth making a new rule looking for a plus and drop it?
Will have to get back to you on the Activity Log...
If you set up e164 processing in 3CX to deal with the +, it will be converted to the proper prefix, depending on where the call is going (international/ national/local). You can then create some new rules (probably 2 ) to deal with the "new" national/international prefix that replaces the +. The current rules can remain in place to deal with number dialled with out a +.
Read through this post, it might make more sense of things...