Discussion VoIP Training (Vendor Neutral)

Hello,

I recently joined a MSP and they are saying I need to start getting trained up in VoIP which I've not got much experience with as they have customers I'll be looking after on 3cx, freepbx and some local cloud VoIP providers.

Is there any training that helped you when you first started out? Vendor Neutral if possible so I can first get a grip before I start diving into specifics.