How to dial special code to enable particular service?

In general if you dial demo code with *24*3471606938#, the phone will take # key as send and only send the characters before # to PBX which makes the calling failed, one solution is on the phone to change key as send setting from # to others.

The other solution is in pbx - outbound route, modify dialpattern to any wanted indentified number for example 0000, set strip to same length as dialpattern, add *24*3471606938# to prepend, afterwards you should dial 0000 to make pbx go through this outroute.