It's not a limitation of your phone system it is a limitation of having 2 x PSTN lines that you can only receive 2 calls.

If you want to receive more calls get a PRI service and a voicemail and you can receive as many calls as you have channels on the PRI and your voicemail will be able to answer and take a message depending on how you set it up.

As advised earlier get a diversion or message facility on your 2 x PSTN lines or get more PSTN lines connected to your phone system.


Regards
Carl