I don't think tho' with the GoTo-statement that calling an alias and then from the alias have the same custom app as outcome will return the call to the position after the originating GoTo-statement (haven't tried this though). I think in the following custom app setting the outcome of alias 9099 back to this custom app will not play the "SorryNoOneThere" msg, but will start at playing "MyCustomWelcome" again:
exten=>s,1,Playback(MyCustomWelcome)
exten=>s,2,GoTo(internal-presets,9099,1)
exten=>s,3,Playback(SorryNoOneThere)
exten=>s,4,Voicemail(5999)
For me this is solved though (for what i wanted to do). I've used your suggestion for the custom app, made a dedicated MAILBOX extension 5999 and an alias 7999 with a ring group and the outcome to that 5999 mailbox extension (not using the LEAVE VOICEMAIL 5999 though, this would keep playing the VMail instructions although they were set to off in Global). Made my own Welcome recording and my own recordings for the 5999 VMail.... works like a charm!
Thanks Selintra and Soprom !!