It will be necessary, yes. There is information passed with the inter-register signaling that is required for call setup.

Here's a good document on E1 R2 theory:

http://www.cisco.com/en/US/tech/tk652/tk653/technologies_tech_note09186a00800943c2.shtml#topic2

Also there's the little problem that the Cisco IOS CLI requires the inter-register signaling type when you define the ds0-group.

Router(config-controller)#ds0-group 1 timeslots 1-15 type r2-digital ?

dtmf DTMF tone signaling

r2-compelled R2 Compelled Register Signaling

r2-non-compelled R2 Non Compelled Register Signaling

r2-semi-compelled R2 Semi Compelled Register Signaling

Some interop documents:

http://www.cisco.com/en/US/products/hw/routers/ps259/prod_troubleshooting_guide09186a0080142533.html

E1 R2 with NEC PBX using 3745:

http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/pbx/interop/notes/246672r2.pdf

hth,

nick