I would agree...VOIP is about the only real option.

But if you need something a bit more robust...

1. Hosted PBX (low cost and works great)
2. Your own PBX (like Asterisk www.asterisk.org)

We run off of a hybrid PBX that allows us to use POT/PSTN (land lines) for the mission critial phone calls and then roll over to VOIP lines (junction networks or teliax).

Although VOIP is great but it can (and does) go down.
There are also issues with latancy (lag time).
If you get above 100ms in your pinging you may have some jitter.

We picked a PBX solution called switchvox.
It allows full PBX with IVR (you know...all those press one for sales, two of support, three to be told off). Then you can have it ring queues (or more then one phone at one time). The first to answer gets the call.

You can also do phone rules based on:

Hours (8-5 do this... 6 to midnight do this..)
Phone numbers (if call is X then ring to this extention)

The cool factor is that you can also run different different companies from one PBX.

Phone line one rings and it gets answered by "thank you for calling company A"

Phone line two rings and it gets answered by "thank you for calling company B"

So much you can do with this.

I use Polycom SIP IP phones.
And I have one phone hooked up at home (standard internet) and it is fully connected to my PBX at the office.

The basic cost is about $1000 for the pbx (includes box) and $100 to $300 per phone.

Any questions let me know.