VOIP for 2 Site organisation with extansions

I'll try to explain this as clearly as possible.
I've been asked to look into the possibility of a VOIP system for a charity. They currently have two locations, due to a merger, one with 8Mbit/512Kbyte ADSL, the other having 24Mbit/2.5Mbit installed shortly (theoretical speeds I know)

Currently, each location has one PSTN line for voice, and another for FAX.

The nature of the work we are now doing means we need to set up an internal communication system between the two sites.

Site 1 has an office with 2-3 workers in
Site 2 has an office with 3-4 workers in

As two organisations became one, and we still have two identities for each side of the work we do, and we have very well known telephone numbers, we do need to keep the numbers for incoming calls on the voice lines (also they're the lines with the ADSL)

What we would like to be able to do, is use VOIP for internal use, with each staff member having an extension or number, and also allow them outgoing calls via SIP.
In addition, we need to be able to route incoming calls to these extensions via VOIP.

I understand there are limitations due to the PSTN lines, so we may have to redirect people to a SIP number - I don't know if a call diversion would suffice (would more than one person be able to call ?).

I'm also not sure on the best route to go for the VOIP system. I understand some of the basics, and we have plenty of spare PC systems of decent spec that I could run a software solution on, but I'm lost on the hardware, or would a hosted solution be best ?

I hope I've given enough info, and not posted a load of rubbish, but as the most "tech savvy" person available, it's been lumped on me to investigate

cheers

Several ways of doing this depending upon how much you want to spend to set it up and to maintain it and the amount of skills available for the job.

Simplest and cheapest to configure is to go for a hosted PBX, where all users connect to a remote exchange. (about £70/month)

The alternative is to look at a small VoIP PBX or Asterisk based system for one end and everybody connects to that. (£500)

If you get the numbers you want to keep converted to DID, then you should be able to get a business class VoIP provider to adopt it. That way you get multiple inbound lines and either office can answer for the other.
You can do it yourself with an ATA at the far end, but to be honest the cost is so low, considering the reliability factor, you are better off paying (Under £10/month).

I would take in consideration how important Voice Communications are to the business. If they are vital, spend the money and purchase a phone system (either IP based or traditional PBX) and make sure you get the support your organization needs. The biggest problem with "do it yourself" systems like Asterisk is that first, it may not meet your needs or may require a level of experience to deploy that might be overwhelming. You get what you pay for, this goes double for phone systems.