MFA secondary call getting stuck in VOIP auto-receptionist

I had the same problem with some of my users who did not want to install company apps on their personal phones. I ended up purchasing a hardware-based MFA token because we also had an auto-receptionist that was not compatible (same as you).
There is a place to configure the source caller ID in Azure here:
https://portal.azure.com/#blade/Microsoft_AAD_IAM/MultifactorAuthenticationMenuBlade/PhoneCallSettings/fromProviders/
But I don't know if this is only for the older Azure MFA server product which is being retired or if it will also work for your Azure AD cloud user. Give it a try and let us know if it worked.
Otherwise, your last resort is the hardware token.
There are many options in the market. The one that I ended up selecting is called Token2, because it was only twenty dollars and is compatible with Azure AD so there is no local server to install.
https://www.token2.com/shop/product/token2-c301-programmable-keyfob-token-with-restricted-time-sync
And then the instructions are here:
https://www.token2.com/site/page/classic-hardware-tokens-for-office-365-azure-cloud-multi-factor-authentication
(The instructions above require Azure AD Premium P1).
If you don't have Azure AD Premium P1 then you need to purchase a USB NFC Burner so you can associate the random number seed on the token with azure as described in the instructions here:
https://www.token2.com/shop/page/hardware-tokens-for-azure-cloud-multi-factor-authentication

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