I have this regex:
If it is OK to have spaces anywhere in your string then it is simple, just add it to your character class
preg_match("#^([0-9 ]+)$#", $post['telephone'])
but this will allow 5 spaces in the beginning.
would be a bit more sophisticated. Starts with two digits, then a optional group starting with an optional space followed by at least 1 digit. this group can be repeated 0 or more times.
This would match
01 23 45 67 89