No as many IP-PBX platforms can register (login) with 2talk as a normal phone would.
In fact this is what many Asterisk users do using the 'register' directive in their SIP configuration.
If you do not have a static IP address or your IP-PBX is behind NAT then you cannot use SIP Trunking and you will need to register your devices with 2talk and use the 'Inbound trunking' feature on each line instead to achieve a similar goal.