What exactly happens when you receive a phone call while running WMWifiRouter (or having the cellular data connection active, really) depends on both your phone and the network you are connected to.

GPRS / EGPRS (EDGE): An incoming call will put the data connection in suspended mode for almost all GPRS based networks and devices. The exception is a GPRS Class A capable device on a GPRS Class A capable network. While this combination allows simultaneous data and voice, availability of both at the same time is virtually non-existent.

UMTS / HSDPA: Simultaneous data and voice is supported. However, some networks still have faulty hardware in their towers which makes this impossible and either data or voice will be dropped when the other is activated. Also, using the registry tweak that makes your data connection automatically disconnect after a set amount of time of no activity disables the support for simultaneous data and voice as well.

CDMA2000 1xRTT: With this type of connection, all calls will be routed to voicemail and the data connection should stay up.

EVDO: The call will come through, while the data connection will drop.

EVDO Revision A: Should allow simultaneous data and voice, but is limited in the same way as UMTS / HSDPA. Some networks have not correctly implemented this and the automatic disconnect registry tweak breaks it.