It's typically not a good idea to have extension numbers start with the digit you are using to access the PSTN in ARS.
You'll need to ensure that 8+555 is not explicitly defined (when its specifically defined, the switch begins routing the call). Instead, having 8+3 to follow will work and get routed either after a somewhat lengthy pause, or by pressing #.
This is coming off the top of my head... If there is another digit you can use for PSTN access that also is NOT being used as a DID leading digit, you may want to do that.