In theory, all VoIP providers use the SIP protocol and should be compatible with VoIP clients such as CLINQ. Unfortunately, some providers prevent registration of their service with VoIP clients other than their own. 

We have collected a list of providers with which CLINQ isn't compatible, as well as other factors that may prevent your provider from being compatible with CLINQ. 

Providers in Germany:

3CX provisioning: Requires SBC tunnel

3CX variousrequires creating a VoIP telephone with as registrar

1&1: registration only possible with an 1&1 IP address

easybell TLS: TLS registrations are currently not supported by CLINQ

Giffnet: registration only possible with a Giffnet IP address

placetel TLS: TLS registrations are currently not supported by CLINQ

Telekom: registration only possible with a Telekom IP address

Vodafone: registration only possible with a Vodafone IP address

Other restrictions: 

PBX: CLINQ requires an open IP address and not a local IP address. As such, PBXs behind local IP addresses are not compatible with CLINQ. 

TCP & TLS: CLINQ current only allows UDP SIP registrations. TCP & TLS registrations are currently not compatible.

Please note that this list is regularly being updated.