I want to link an office of my firm with another which is in Germany through the Internet without implicatting any specific VoIP providers, just my net provider.Can I do that just buying the necessary equipment ?
Well.. good luck. Remember that voice is a dely and packet-loss sensative medium. VOIP is fine over PTP T1s, but even MLPS networks have issues when the providers are 'guaranteeing' you priority and <1% packet loss. As far as hosting a real VOIP trunk over the internet to another continant, you would probably experience so much jitter and chopping it would be more frustrating than anything.
On the other hand there are 'internet phone' providers such as Vonage... I do believe they have business services that would be condusive to your needs. Your calls would be going across their PTP trunks, but as far as priority and packet loss, still not guaranteed outside their network. But you still do have a phone bill at the end of the day.
So, if you have a PTP data infrastructure you would like to leverage to run your international calls over, very do-able.