General aviation pilot preparing for departure at CLS reported communication with Seattle Approach was extremely poor on the ground and remained poor even after departure.

Narrative

Pre-departure communications with Seattle Approach are poor for IFR departures from CLS. The frequency listed is 121.10. However; it took multiple attempts on the ground to contact on 121.10 before anyone answered but even then; reception was weak and we were told it was the wrong frequency and to contact Seattle on 124.95. But no one answered on 124.95; (or on 126.50 or 124.50; the frequencies we had used a few hours earlier for the IFR arrival to CLS). I then used my cell phone to call Seattle Center only to be told it was the wrong sector" with no offer to connect me with the proper sector. Then back to the radio and 121.10 where that controller agreed to relay the IFR release clearance.The clearance we received: Cleared to ZZZ; as filed (filed routing was MCKEN VXXX.ZZZZZ ZZZ); climb and maintain 5000; on departure contact 124.95.Once airborne communications were still poor for the first 5 minutes. No communication at all until climbing through 2500 feet; and even then; communications with Seattle were broken. This; (combined with another aircraft on frequency with a similar call sign?) resulted in confusion with a controller issuing us an illogical routing to a waypoint (BANDR) not on our flight plan or anywhere near our filed/cleared route and handing us off to 119.2; (the wrong sector for our filed route of flight). The controller on 119.2 immediately asked us what we had been told; (apparently surprised by us on his frequency?) and then we were then returned to the previous sector where the controller apologized for having been confused about our destination.All in all; a very poor communications experience both on the ground and post-departure airborne. I don't think this was a problem with aircraft radios as we haven't had problems like this anywhere else."