You need to provide logging from Asterisk. I’d suggest the full log (which needs enabling) after doing pjsip set logger on.
However, I think you are confusing extensions with devices. 550 is a device.
This might be because you are using FreePBX, which conflates extensions and devices. IVR has a specific meaning in FreePBX, whereas, in Asterisk, it is a generic telephony concept which could be implemented in many different ways.
Typically, for FreePBX, I believe you would use:
Goto(from-pstn,550,1)
for a virtual extension accessible from a PSTN call. from-pstn could be replaced by some other context.