GAP compatible DECT phone handset can be used for Siemens, if I read the specification correct.
Theoretically, the Siemens can handle 3 concurrent calls. In you case, when you use the DECT phone attached to SPA3102 to receive the PSTN call, You can only have ONE concurrent call. In the best case, you can have 1 PSTN call and 1 SIP call.