Startups with offices: what about phones? 19 years ago
VoIP + Asterisk is the way forward. With a cheap linux server and cheap SIP phones or adapters you can easily do nearly everything an expensive phone system can do if you really need it.
Asterisk does need a bit of setup but it's not that hard for someone technically minded.
BTW, If you do any kind of conference calls get a proper conferencing phone, they are kinda expensive but so much better quality than speakerphone on a normal phone.