Please check Google or AppStore for recent SIP softphones for iPhone.
E.g. this article describes
SIP phone should be configured using generic SIP parameters as described on SIP settings page.
You can use the same SIP softphone application as for iPhone.
iPod touch doesn't have built in microphone. You can build your own microphone for iPod touch from mobile phone handsfree set. This article explain in details how to do it. DIY iPod touch microphone