Hi Kieran, here are my responses:
Q: My understanding is that Linux DHCP scope management is not a present capability of the system. From a quick look, I couldn't see it being on the roadmap - is this correct?
A: I can confirm that Linux DHCP is currently not supported. You can vote for this feature here - http://thwack.solarwinds.com/polls/1049
Q: The telephones don't respond to SNMP so i'll need to use ICMP pings - will this provide MAC address information back?
A: No, but see my answer to the question about neighbor scanning
Q: The switches that the telephones are plugged into will be monitored through NPM. Will I therefore need to manually define them in IPAM in the Neighbour Scanning section again or will it be automatically transferred across
A: You need to set those routers manually in IPAM.
Q: What benefits will I gain from enabling Neighbour Scanning if I can ICMP ping (but not SNMP poll) the telephones?
A: You will get MAC for IP addresses
Q: Is there anything else I need to think about?
A: I would recommend downloading trial version of IPAM and try it in your environment, this is probably the best way how to verify, that it fulfills all your needs.
Thanks for the info - much appreciated.