There seems to be some serious confusion here, let's see if we can clear things up.
What does "support VOIP" mean here? Strictly speaking, any device that can handle IP packets "supports VOIP", so OpenWrt qualifies. After all, VOIP means "Voice Over Internet Protocol", and OpenWrt obviously supports the Internet Protocol.
What's a "VOIP port"? Any Ethernet port can be used for VOIP. Or do you mean a port that has Power over Ethernet, so your Vodafone can be powered off a single Ethernet cable?
If you are using wireless to successfully connect your VOIP phone, why are you asking about a "VOIP port"? Do you want to use Ethernet instead? Have you tried connecting the phone to an Ethernet port on the OpenWrt device?
Because Raspberry Pi wireless is completely ill-suited for use as an access point, which is apparently what you're doing here.
Same reason as before, the wireless is ill-suited for use as an access point. That device is designed to connect a client (like a PC or a laptop) to a wireless network provided by a dedicated access point, not the other way around.
Actually why are you using these wireless parts anyway? Does it not work if you connect your VOIP phone to your regular wireless network? Or like I asked earlier, to one of the Ethernet ports on your OpenWrt device?