IP-to-Tel calls are SIP calls which are forwarded from the IP network into the public telephony network. SIP calls which are forwarded to ISDN or analog phones locally connected to SwyxConnect 5000/8000, are also IP-to-Tel calls.
Calls which are forwarded from the PSTN (public switched telephone network) into the IP network via SIP are Tel-to-IP calls. The same applies to calls to the SIP-IP network from ISDN or analog phones connected to SwyxConnect 5000/8000.

IP-to-IP calls are calls that take place within the SIP network and do not proceed to the ISDN or analog network, e.g. a call from a SIP client to a proxy (and finally to another SIP client).