First some numbers. Depending on codec used but the best codec G.711 requires 128Kilobits per second for each VoIP phone's conversation (i.e. 64Kbps in each direction) to ensure Quality of Service (QoS). Assuming 30 VoIP phones are used concurrently at times, a total bandwidth of 3.84 Megabits/s would be needed. Your nominal bandwidth (100Mbps) is more than enough to allow for QoS on VoIP, many times over.
However, I'd say the potential latency in your case (with the VoIP server being located far away in Israel) may be more of an issue for your VoIP quality than your Internet speed. Try pick a server in Israel and do a speedtest. That would give you a more realistic latency affecting your VoIP traffic. Latency higher than 300ms is generally unacceptable for reasonable conversations, especially for business organizations.
As for implementation of QoS on MIkroTik, it is a three step process: Classify->Mark->Prioritize. The linked article below gives an example in implementing QoS on MikroTik. You can modify to suit your VoIP.
http://networkgeekstuff.com/networking/ ... n-example/