At first i wrote "we cannot find your telephone number" then I changed my mind as i remembered that for me DL usually wants a conjugation of "poder" in the spanish phrase before allowing the use of "can" in the translation. "we did not find your telephone number" was accepted.
Duo seems to be intentionally omitting poder in this kind of sentence/question, but still translating as "can't." See the "tips" for the School 4 lesson-- https://www.duolingo.com/skill/es/School-4/tips (keep scrolling down to encontrar and encontrarse). Just something else to learn, apparently!
This sentence can be either present tense or past tense.
The Spanish simple present tense and preterite tense is identical for the "we" conjugation of regular -AR and -IR verbs.
As far as adding "can", that is Duo's clumsy attempt to teach English speakers to infer implied words like "can, will, should" that are not actually stated.
I'm having trouble with that one.