We have a customer that we are delivering a PRI (transport is fiber to a Cisco 3400 and then in to the 908e w/SBC) to our subscriber and he has asked about turning up a SIP trunk in the same 908e so he can test SIP in his Asterisk PBX he just purchased.
While it seems quite well feasible to do this, has anyone done this (either in a lab or real world scenario) and were there any huge obstacles to overcome?
Yes, several times. No problems.
Some previous comments here recommend "accept $ cost 0" in grouped-trunk voice configurations. You'll want to avoid that here (and it's kind of sloppy and a bad habit anyway). Define the DID range(s) for the PRI accepts in a grouped-trunk configuration, and include the PRI trunk in that group.
Create a new trunk type SIP for the Asterisk. Create a grouped-trunk for it and include the new trunk. Then within that grouped trunk include the DID(s) that should terminate on the Asterisk. You can "borrow" some from the PRI or route different DIDs entirely. Don't forget media-gateway IP primary on the interface for the Asterisk as well as that for the external SIP server back to PSTN.
You may need to normalize dial plans with DNIS substitution patterns if the different devices format things differently. Add local NPA to 7-digit calls, add or strip e.164, etc.
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