it is possible to use Sip gate on any network as long as it meets the minimum requirement for VOIP. (mobile network 3g or higher)
you could just use a App on the mobile, to connect a dect you would use a dect that is sip completable, you would have to connect it to a router that is able to use the WIFI as a wan connection. another option would be to use a Sip ATA, you could also get a router that supports VOIP again it would have to support a WIFI wan.
its not difficult but could be a learning curve
i can not guarantee you will not have carrier grade NAT or double NAT issues
Edited by copex (Sun 11-Oct-20 19:32:09)