no support for USSD codes

Plasma Dialer has no support for USSD codes like *100# (debit check on prepaid) or *#06# (query for IMEI).

When a USSD code is entered, dialer errors: "Unable to make a call at this moment".

Tested in plasma-dialer-git 0.2.r0.gf2f6865a-1 on Manjaro unstable (Beta2 + updates) on a Pinephone. Carrier is Congstar (sub of Deutsche Telekom, Germany)