We are looking for a VOIP solution and we have the following requirements:
1. To be able to forward call into the organisation - to other departments for example.
2. To have a "hold" function.
3. To have a voicemail function.
4. The equipment to have the option to work with headsets.
5. Software phone
The idea if the solution needed is to work like phone central, to be able to accept, transfer multiple calls.
Here is a list of things I'd recommend doing in order to help you on your search:
1. Use a broker. It will save you a lot of work and help you avoid mistakes.
2. Make a list of must have features (i.e. Do you want full Lync integration, do you want mobile applications, do you want voicemail to text translation, do you need to customize your music on hold, do you need real-time call reporting for mangers, do you need an overhead pager system to be integrated, etc. Some providers will have these features and some will not.
3. Know your calling usage data. Some providers are great for heavy callers and some are not.
4. Know your seat types and seat count (i.e. how many regular handsets vs. common-area phones vs. conference room phones vs. softphone or mobile-only stations, etc). If you have a lot of each, some providers will have more customized options and will charge you less for certain seat-types, while others have a one-size-fits-all.
5. Choose providers who can deliver a private circuit for sites with 10+ users. Not all providers can do this but this will guarantee call quality.
I hope that's a good start!
That's just a few items but attached is a more detailed article that I wrote on 8 detailed shopping hacks for hosted VoIP.