@Mkh 

If you look at your sound control panel where it lists all the available playback devices, do you see the headphones there? Maybe enabling the device or setting as a default device.

Have you tried disabling the build-in Bluetooth just to see if they were getting in the way? Make sure you are comfortable in doing this so you don't do it and not able to enable it.

@Tony 

1) The headphones are not visible on playback devices. 

2) Can you elaborate what do intend by disabling  "inbuilt" bluetooth.

Besides, I did a device manager comparison.

- On my lenovo T470, PXC 550 are registered as Microsoft Bluetooth enumerator.

- Whereas with UB400 on my desktop, they are registered as Low energy device ( Microsoft Bluetooth LE enumerator). I tried to disable the LE enumerator , but the headphones do not pair. Also I donot see AVCRP for PXC 550.

It seems UB400 does not accept / match the bluetooth profile of PXC 550. Even though they connect flawlessly with mobile phones, my laptop.

--

@Mkh 

Disabling the Bluetooth was to see if there was any sort of interference from the device. 

The only Sennheiser headphones that were tested were the HD45s, but not the PXC550s.

I'll keep looking into what I can find out.

@Mkh 

Unfortunately, it looks like they may be incompatible.

@Tony 

Thanks for confirmation.

Is there a way we can Identify what is the source of incompatibiltiy. As long as it is not hardware related, I'm hopeful can be fixed.

To me it seems the UB400 is not able to interface with PXC 550 and accept its audio profile.  I did try to scan through event log to find where and what the mismatch occurs. I'd be happy to help if someone can guide me to a log / powershell script to establish a connection with PXC 550 to know the exact error.

I'm sure once found, should be an easy thing for UB400 firmware dev to impement the compatibilty and fix this once of all.

.... unless it's something hardware related. 

@Mkh 

I followed up with HQ and they mentioned the incompatibility was in fact hardware related, unfortunately.

@Tony 

Ok. Thanks for confirming.

@Tony Is it known whether the UB400 works with PXC 550-II?