In this article I’ll go over how to build a SIP Trunk to a provider with Call Manager Express, in this case we’ll use Flowroute. I will be using my 3725 with IOS “c3725-adventerprisek9-mz.124-15.T10.bin”.

First we will enable SIP, then we configure the router to register with our provider, then we’ll configure our codecs and dial peer, finally we’ll go over some show commands to verify that everything works.

Here’s how to enable SIP:

voice service voip

Very simple, we just enable the SIP protocol.

Next we configure our router to register with Flowroute:

 authentication username xxxxx password 7 xxxxxxxxxx realm
 calling-info pstn-to-sip from number set 1xxx7325736
 no remote-party-id
 registrar expires 3600

This tells the router to register with “” using the username and password we specified.

Next we configure our codecs:

voice class codec 1
 codec preference 1 g729r8
 codec preference 2 g711ulaw
 codec preference 3 g711alaw

Here we specify which codecs we want to use and what order we prefer them, 1 being most preferred and 3 being least preferred. I have chosen to use g729 whenever possible as it needs much less bandwidth and sounds great.