Huawei Nexus 6P :: Dialer Won't Allow SIP Calls (Claims Call Barring Is On)

I'm trying to use the Nexus 6P's native dialer to make calls through my SIP provider. I have a configuration that works perfectly on the Nexus 5X and older 6 (shamu). I entered all my information in Dialer > Calls > Calling accounts > SIP accounts, and then enabled the account under ... > Calling accounts > All calling accounts. However, whenever I attempt to make a call through my SIP provider on the 6P, a message pops up saying "Can't make outgoing calls while call barring is on."

Note that: I can make regular phone calls from my mobile number just fine, tested with both a Verizon and a T-mobile SIM. However, the SIP call barring problem persists regardless of which SIM is in the phone.

I can make SIP calls using other apps, like CSIPSimple, so it's not a port blocking issue.

I can receive incoming SIP calls (to my DID number) just fine when enabling ... > Calling accounts > Receiving incoming calls, so clearly I didn't mistype my SIP account password. (I visually checked that all the other settings are identical between my 5x and 6P.)

I'm definitely on WiFi when I experience the problem. To be extra sure, I even tried it with airplane mode and WiFi both on, which is fine for CSIPSimple or incoming calls but still doesn't work for outgoing calls with the android dialer.

The message pops up instantly, making it seem like a setting on the phone rather than a response to network messages.

For what it's worth, I'm running build MMB29M (android 6.0.1). My phone is unlocked but not rooted. (I can still get root access to /data by booting TWRP, but have never mounted my system partition read-write.) The one thing it might be is that I put a Verizon SIM in my Nexus 6P when first booting it, whereas my other phones were all first booted with a T-mobile SIM. I think that caused some VZW apps to be installed that I since removed; perhaps it messed up other settings. I'm a bit reluctant to wipe my phone and try again, but will do it if it's the only solution.