For ease of management it is usually better to purchase a VOIP service (ITSP). A lot of time when purchasing a hosted service, they will provision the phones for you and you do not really have to configure anything.
Voice service options will range and the prices vary of course. If you are supplying the phones and the ITSP is supplying the service, typically the provider will supply you with a proxy, user id and password which you manually configure on each phone. If the provider also provides the phones, they are usually pre-configured and plug and play (generally speaking).
The SPA9000 is EOS/EOL for quite sometime and was replaced by the UC320. To my knowledge the SPA9xx phones are not supported by the UC320. So it would seem the hosted service would be a good route for you.
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