It's likely that there will be a 1x1 mapping between Employee and voice Telephone numbers. So, keeping them in one simple table could be OK. Fax numbers may be another thing. Also, telephone numbers can usually be kept as integers which only take up 4 bytes, though you'll need an algorithm to split them up for display into country code, NPA, NXX etc.

OTOH is there any other info you'd likely keep along with the telephone numbers? e.g. when the number is valid (start and end dates), when time of day or days of the week you can call someone at that number, etc. When things get a little more complicated, it's might be cleaner to have a telephone number table with a FK to the employee id, a number type (landline, mobile, fax, etc), and other things.

Basically, the wider a table gets, the more you should look for opportunities to normalize into two or more tables. It doesn't mean you have to do that. It's just an indication that you should look at it.

The exception would be a Data Warehouse, were denormalized tables are the norm, for faster reporting.

There's not one "correct" answer to your question. Like so many things in the DB world, "it depends".