We're talking about two different products here:
3CXPhone6 (the one we discontinued about five years ago) was/is a generic SIP phone, able to work with any SIP-compliant VoiP provider or PBX (including 3CX); we also had similiar clients for Android and iOS.
Then, starting with 3CXPhoneSystem 11-12, these clients were discontinued and were "branched" as different, 3CX-bound phones, working alongside Call Assistant (later MyPhone). To ensure designed functionality, these clients are the ones checking for PBX user-agent, not the 3CXPhoneSystem. 3CXPhoneSystem PBX continues to work with any SIP-compatible phone - 3CX, Snom, Yealink, Counterpath, Acrobits, Grandstream; it performs client's user-agent checking only for the non-sip, 3CX-specific functionality, like MyPhone and push notifications.
Hope it's clear now.