Hello. Set the task to migrate your analog telephony to IP office somewhere in the 10-12 customers (3-5 lines would be sufficient for now) with the possibility of extension. The main requirement - ability to use telephony as with phones and ip phones. I saw somewhere things like sipnet'a when running in the cloud PBX and the phone works with computer, but is it possible in case you need to connect it to the phone?
Multichannel number with access via a sip now readily available. And to connect analog phones using sip gateways (there are options for different number of ports for phones). Accordingly choose a provider, just in case consulted with him on compatible hardware, buy a gateway(s), set up and connect.