Hi Vasili, thank you for the picture you sent through facebook. Yours is a Siemens & Halske telephone from the early 30s. Not a W28 (the handset cord does not enter from the left side, but from the back).
The exact type should be on the bottom. Probably VSa Tist 66 followed by a letter. This phone is from Bulgaria and the writing is from communist times. Is there a coat of arms on the back?
The exact value is hard to tell. It very much depends on the condition.