I use the Linksys PAP2T-NA, and they work well for me.
However, I may suggest using the SPA-2102-NA because it includes a NAT-Router too. It may be easier to configure for QOS (quality of service for VoIP) and may automatically take care of VoIP port forwarding issues.

With PAP2T-NA, you will need to know how to "Port Forward" or "Port Trigger" SIP ports 5060-5063 and RTP ports 16384-16482 on your NAT-Router. This is important for VoIP to work properly.

You can configure two separate VoIP services with PAP2T-NA.
I configure mine for CallCentric on line-1 and InPhonex on line-2.

Just follow the configuration instructions of your PAP2T-NA or SPA-2102-NA adapters as documented by Linksys.

Also, your VoIP service providers like CallCentric and Inphonex will provide information on how to configure your login and password into your VoIP ATA adapters.

Your PBX shouldn't have anything to do with your internet service. You can connect the PBX to your computers, but the phone and internet wiring are seperate.

Hmmmm, I'm not really familiar with the IPhone Wi-Fi but my friend has the same problem but for him it doesn't seem to connect. First of all if you secured your network using WPA2 its harder for devices to detect and connect. Try switching browsers(if that's possible). It could also be a manufacture defect.

The best way to solve your problem is going to an Apple store and asking them for help as they have the most expertise.

Go to there website they may have a solution for you there.