What's the best server configuration (hardware and software) for my company?

So, here is the question: I started to work for a small company, where the system is a bit weird... 13 workers, developers mostly, two servers and a lot of cables. I would like to know what would be, in you opinion, the best installation for my server room.

Here is what we already have:

• 2 servers running Windows Server 2003 (1 for communications, 1 for the developers work) --Software-- *ESTOS ProCall for telephone *Tobit David for Email *A few internal programs and a lot of files that need to be accessed
• 1 double-wan router
• 2 switches
• 10 telephones
• 12 workstations (all Windows XP or Vista)
• 4 printers (for different applications)
• 2 Notebooks for external work

The most important for us is to have a good telephone ans email system, what is not really the case at the time. We use ISDN because of a better quality, but the whole software compatibility between David and ProCall is bringing on some errors, and I think it's not normal to have to reboot a server every 2 days... I would like to have suggestion for a "perfect" server room for us. I was thinking of making the communication server work on Linux or MAC OSX Server? For geographical information, the company is in Germany. Low costs would be good.