No, in such a small environment you could happily be running your phones and other devices in the same ip-space or vlan.

There are however many considerations to make, for example if the pbx talks dhcp to it's phones to handover parameters you could perhaps consider investing in a managed switch to seperate the voip from the other-clients environment. Also, if you expect your network to grow, it's better to do it right at start and design your network for growth.