Creating Apple ID for Business MDM Purposes

I'm hitting a wall here and trying to figure out best practices. I run a managed service provider, focusing mostly on Windows and macOS devices. Some clients have expressed interest in us managing their mobile devices. We already have an MDM and have done this already less than a handful of times. In most cases, we've used the Apple Business Manager process as the client makes all their purchases through Apple directly. We have two new clients who make their purchases often through 3rd party resellers (refurbs). In the past when Apple Business Manager isn't going to work and we have to take a BYOD approach, we've created a new Apple ID specifically to manage the push certificate required by the MDM. It appears, however, that my personal cell number that we often used for this purpose has reached the hidden limit of 6-10 Apple IDs and cannot be used anymore. Short of buying a new number through a VOIP provider (and hoping the SMS works, which isn't always the case), what options are available for this scenario? I can obviously use my own Apple ID to create certificates, but I'd rather have something in place that the company has ownership over. Any ideas?