Actually, either one is correct depending on the circumstance. The first part of both sentences ("I tried to call you") is clearly in the past tense. The second part of the first sentence ("but your phone is disconnected") is in the present tense, meaning you are talking to the person right now and telling them that as of now, their phone is disconnected. The second part of the second sentence ("but your phone was disconnected") is in the past tense, meaning at the point in time you made the call, their phone was disconnected. The second sentence is "safer". Since you most likely do not know if their phone was reconnected by the time you spoke with them, leaving the entire sentence in the past tense is a 100% accurate statement. The first sentence is "less safe" but potentially more "caring". You do not really know if, at this moment, their phone is disconnected. However, by using the present tense you are implying that the phone problem is ongoing and they should try to get it fixed.

My comment is that you should not place a comma before "but". This is a conjunction using to connect two sentences together with it in the middle. A comma is used as a pause. If you need to use one, then your sentence is too long and needed to break up.

The second one is correct. "I tried to call you" (past tense) "but your phone was disconnected" (past tense). "your phone is disconnected" (present tense)