You ask some good questions. I can answer some of them.
Traditionally, the phrase "trunk" refers to a T1 or PRI or similar line that can handle multiple calls at the same time. This term was adopted for use with SIP. Some people say that "SIP trunk" is merely a marketing term. In essence, it refers to calls delivered from one point to another via SIP. To answer the question from your subject, I think it would be correct to say a VoIP service provider could provide you with a SIP trunk, or to use other words, could provide you with incoming and outgoing calls via SIP.
said by Cloneman:- What could go wrong with using a VoIP provider with a PBX?
Security is the primary issue that comes to mind. Hobbyist PBX devices are targets for hackers who want to route calls via your service provider on your dime. It is certainly possible to secure a PBX, but it will take a moderate amount of studying to learn how.
I think a secondary issue is that your PBX becomes another point of failure. If it has a hardware or configuration problem making it inoperable, your phone won't work.
said by Cloneman:- What's could go wrong using a "SIP Trunk" provider directly on a IP Telephone?
Many of our forum members use IP phones without a PBX. This is a relatively common thing to do. If you don't require the features of a PBX, or if the PBX features of service providers like VoIP.ms and Anveo meet your needs, it may be easier/more reliable/more secure to work without one.
m.